Updated - 3/6/15
This post is not about brick and mortar stores in the garment district; rather, it focuses on the fitting systems used by industry professionals to make garments that fit. These are names known and trusted by many of the firms you will find in the garment district. You can go to brick and mortar stores to get dressforms, but you'll want to do some homework first. If you sew for yourself, you may find that fitting yourself is a great challenge. Most of us don't have access to qualified, patient friends to help us with fitting.
So... what can you do? Here are some options: Find yourself a dress form. The most affordable, which you can use for fitting, but can be problematic for design work or anything more complex, is the Twin-Fit brand, which you can find, among other places, at SIL Thread. At this moment, you may expect to spend somewhere near $200 for one. Not the best choice, but a workable one. If your weight remains relatively stable, and you would like to make the investment in a form you can sink your pins into, and use for draping, get one directly from Wolf Forms, Ronis Brothers, Sewmark, or Andy's. Expect to spend $600 or more, depending on your needs. Wolf (mentioned above) can make a custom form for you, based on your measurements, but it will take time, and a visit to their New Jersey facility to have this done. If you have a significantly unique figure, anatomical challenge, and a healthy budget, this could be a good idea for you. Expect to spend in the vicinity of $1200 for a half body, or $2000 or so for a form with legs, custom made. Considering that people will pay this much for a custom garment, this is actually a pretty good bargain.
If you make things to photograph and sell, you can't beat Bernstein Display. You buy a great display form, and actually make your form part of your decor, if you get one from this company. So let's say you can get your hands on a used form, or you already have one that isn't exactly your size (hopefully smaller in some places than you are).
Consider Fabulous Fit, a Brooklyn-based company you will only find online. With the help of some wonderfully shaped pads, you can build up the chest, waist, hips, rear end... wherever you need to add. An extremely affordable solution. (about $90)
You can certainly buy used forms, but there are some deal-breakers to be cautious of... A for that has sustained water damage A form that has been overused, and is now fragile, ripped up, or concave in places. A broken or off-kilter base. (because no one wants to fix them) A very old form (because the proportions are for a woman who wears very restrictive undergarments - unless you still do that...in which case, I am both impressed, and maybe a bit concerned for you...) Finally, of course you can make your own form, as shown on YouTube videos (easily searchable), but it is not easy, and will require some materials... so I would say it is at least $200 worth of effort... so why not buy a Twin-Fit instead? Armed with this info, I wish you all a very happy fitting experience.