When I encounter someone visiting a film set for the first time, I can almost guarantee they will say something like "I had no idea there would be so many people! And they're all working!" I've heard it on big studio productions, and I've heard it on little student films with skeleton crews, relatively speaking.
The latter end of the scale is a necessity of minimal budgets, of course, but sometimes filmmakers let those crews get a little too skeletal. And I'm not entirely convinced that is a necessity. People doing double or triple duty is one thing. Entire areas of responsibility left to more or less fend for themselves is quite another. This list is in no particular order (mostly because the impact of the lack of someone doing a given job will vary by production), and by no means guaranteed complete, but these are the gaps I've noticed time and again.
Costume Designer. Okay, I said "no particular order," but this is the one I would probably be doing if I weren't an actor, so I get soapboxiest about it. Clothes do not just happen. Your clothes, what you are wearing at this moment, did not just happen. You might or might not have thought about the reasons you bought those particular pieces (or why someone bought them for you), and why you pulled them out of the closet or drawer this morning, but there are reasons all the same. Making a film is creating a world and the people in it. In order to do that, someone needs to figure out what clothes those people would own and wear, and why.
In the microbudget world, you're probably not making a detailed period film (and if you are, you'd better have at least a year to set aside for pre-production -- Team Witchfinder, I salute you!), and you're almost certainly not looking for someone to build a bunch of clothes from scratch. What you need is someone who pays attention to clothes and what they say about people, knows how to shop (especially thrifting), and preferably has a good sense of color. If you have no budget for wardrobe, that same knowledge is what they need to guide the actors to plunder their own closets effectively. Because I'm sorry, directors, but the vague guidelines I get the vast majority of the time? Rarely result in anything resembling the image you have in your head. And I know this stuff. A lot of actors don't. Directors don't have time -- or, often, knowledge -- to work out all the details of how to make a look happen. That's why they need costume designers.
Separate Director and Producer. That thing I just said about "directors don't have time?" It's going to be a theme here. There are exceptions to every rule, and if you're super-detail-oriented and driven, you might be one of them. But chances are there's a whole swathe of business and organizational stuff you dread having to deal with. That's because it's supposed to be someone else's job. Especially if everyone involved also has a day job. Riding herd on all the artistic stuff and the business stuff AND a day job? Will send pretty much any director to the hospital. That's why they need producers.
Script Supervisor. Another one that tends to fall to actors. Which, aside from the energy drain of keeping track of our own continuity, is just supremely uncomfortable in the chain-of-command sense. When everything is ready to roll and everyone is looking at me, I feel super-extra-squirmy holding up my hand and saying "Um, wasn't his jacket zipped up before?" or whatever, because it's not my job. It is, in fact, perilously close to telling the director how to direct, which is the biggest actor no-no in the universe. But unless a director has a flawless eidetic memory, sooner or later some detail is going to fall through the cracks in the course of making the big picture happen. That's why they need script supervisors.
Grips and PAs. Lots and lots of grips and PAs. Unless you are physically tripping over people in every direction (and, depending on how close the quarters are at your location, sometimes even then), there is no such thing as too many hands. Some directors take pride in participating in the grunt work, and that's awesome. I applaud that. But they can't direct the actors and hold a reflector at just the right angle and fetch fresh batteries and hold coats for actors pretending it isn't 30 degrees and make coffee and keep the gawkers out of frame and turn the noisy heater on and off and fetch extras from holding. That's why they need grips and PAs.
Makeup and Hair. Insert a lot of the same stuff I said about costume designers. Just because you don't need special effects doesn't mean you don't need knowledgeable people in charge of what your actors look like. Sure, most actors go around every day looking reasonably presentable, but most of us know sod-all about how to translate that for the camera, or how to look a way we normally don't. And a smart director knows that -- as with costume design, and a whole lot of other things -- it takes skill to present something in a way that the audience doesn't notice or think about it. That's why they need makeup artists.
But where do I find these people? If you have next to no budget, it's a serious question. In which case, if you don't already have one, you really need to start with that producer. That's when it's important for that person to be good at finding answers to things. Like "What does the person doing this job need to know?" and "Where should I look for someone who knows those things who might be willing to pitch in?" If you know people, you probably know people who can do these jobs. Organized and detail-oriented? Ask them to be script supervisor. Makes the rounds of all the thrift stores in the area once a week? Sound them out about costume design. Somebody's cousin is a Mary Kay distributor? Talk to them about makeup.
And of course there are scads of indie filmmaking forums, Facebook groups, and so on. On many of them, any post actually looking for crew (as opposed to spamming self-promotion) will get noticed and get responses. Obviously it's better if you're paying (even if it's just a modest stipend), but if you genuinely absolutely can't pay anyone (most definitely including yourself), there's someone out there who'll be interested in doing it for fun.
In all seriousness, if you can't find people to fill all the jobs a film really needs filled, I urge you to seriously consider whether you should be making this film right now. If you wait and do it right, if everyone has a great experience on your set because it actually felt like a team working together to make an awesome thing, if it's not a desperate shorthanded effort to just get something, anything, in the can so we can all go home and put this nasty business behind us? Not only will you have a better film at the end of the day, but you'll have a bunch of people who would work with you again in a heartbeat, will refer their friends the next time you need crew, and will proudly support and publicize your baby.