I can't for the life of me remember now, but last week I was talking to someone here who was looking for cheap but good corsets and I promised I would look up where my dance group gets ours. If you were that person, or anyone else that wants a corset, I got the site for you!
I have a few of these - mostly the ones that are called "Twin Button Corset" like the pink one pictured below that come in multiple colors, and the quality is good and you can do a lot with them. They do run small but the laces in the back give a lot of room for leeway, and the nice thing about them is that they actually zip up the side, so you can lace yourself in and get the size right and then knot it and use the zip to get in and out, so you don't have to re-tie every time.
The only other thing I'd point out is that shipping at the site is usually ridiculous - I would double the price of the corset you see listed to get the final overall price. However, even with the crazy shipping they are still super cheap - for instance, the corset below is 8.85 and with shipping comes out to about 15 dollars. If you can order far in advance or if you're only ordering a few things it can be cheaper.