This will probably sound weird, but I collect dresses. Ever since I was a freshman in high school, and I discovered that I absolutely love dresses, and I love being able to put them on and go somewhere. Now, 8 years later, I have a collection of somewhere around 75 dresses. About 45 of them are prom style dresses, and I even have 2 old fashioned wedding dresses.
I'm sure you're probably thinking that I'm rich, and that's how I've bought all these dresses, but you're wrong. The most I've spent on any of these dresses is $20. No, that's not missing a zero at the end. I've never gotten a dress, even a prom or wedding dress, for more than twenty dollars. In fact, I didn't pay anything for either of my wedding dresses. No, I didn't steal them either. I just know the right places to look for cheap prom dresses, and I can help you find some too.
First things first- we'll start with finding free prom dresses. Now, there are two different sources for free dresses. The first we'll call "networking", and the second is online. Networking will most likely be the easiest way to get a dress free, but it will also require the most luck. The way you do this is by telling everyone you know that you're looking for a dress, what size you're looking for, and anything else important. You will be amazed how many people will either say "I have the perfect dress sitting in my closet." or even "I know someone who was looking to get rid of a dress just like that." It won't always happen with the first person you ask, and it won't always happen at all for some people. But it's one of those things that will never hurt to try. I've gotten about 6 of my dresses that way, including the one I actually wore to my senior prom.
The next method is online, where you can still manage to get free or cheap prom dresses. The first thing to do is go to http://www.freecycle.org/ and sign up to receive emails from the groups nearest to you (only go as far as you'll be willing to travel) Be warned that, depending on how popular Freecycle is in your area, you may be getting a lot of emails. Also remember that, like with any method of getting cheap or free dresses, there is no guarantee. The first email you get could be exactly what you're looking for, or it could take over a year before you come across the perfect dress. That's why you have to try everything. Another good website to watch is http://www.craigslist.org/ If you watch the "free" and "clothing/apparel" sections, you can sometimes find people who are giving away old dresses, or selling them for a cheap price. For both websites, make sure to check frequently.
Now that I've shared all the methods I know of getting free dresses, and a few possibilities for getting cheap ones, its time to continue. The next thing I recommend is going to thrift stores. Dresses, even fancy prom dresses that originally cost hundreds of dollars, and may even still have the price tags on them, are rarely priced at over $20. The important thing is to know how to look. Avoid any dresses you see displayed- those will almost definitely cost more. Look through the "dresses" section, but if you aren't seeing anything fancy, it is entirely likely they have a separate section for "formal dresses". If you find a dress you really like, even if it still has the price tag on it, inspect is closely. Check the hem for stains and tears, and look at the whole thing in good light to make sure there are no stains anywhere on the dress. Any issues you find, figure out if they are fixable. small tears can usually be fixed fairly easily, as can places where stitches are simply pulled out, or a strap has come off on one side. There is no way to know if stains will come out, but some may be able to be covered, such as with a broach or ribbon. Once you've found a dress and determined that there's nothing wrong with it that can't be fixed, the next step is trying it on. It's always best to have someone with you who can help you into and out of the dress, and give you an honest evaluation of how it looks. Make sure that you can stand, sit, and walk in the dress. And most importantly, make sure you like it.
If none of these tips have helped you, a few other places to try are consignment shops, and clearance sales in dress stores and online. Good luck finding your perfect dress!