Fabric
Corsets can be made with any fashion fabric on the outside (i.e. Satin) but should have a suitable strength layer. I prefer coutil, though a high-end twill or drill is also sufficient. That being said, good luck finding a high-end twill at your big-box fabric store that doesn't stretch out like a pair of tight jeans from dryer and feels baggy 30 minutes later. Most professional corset makers will use coutil.
Boning
You have 3 types of boning: Rigilene/plastic, steel, and spiral. You can buy the Rigilene stuff at your local fabric store and it's generally intended for bodices and strapless dresses. Under pressure the Rigilene will bend and warp, but it's also easy and inexpensive to replace. The flat steel stays seem to be the most common in commercial corsets - but I've seen those become permanently flexed in a short amount of time. Spiral bones are the best, but are the most expensive and require special equipment to cap the ends before using.All that being said, once you drop a zipper in the back of a floorshow corset it doesn't really matter. Zippers are not designed for horizontal stress any more than the aforementioned Twill fabric. So now you pretty much have a lace-up bustier. And if you're going to drop a zipper in a corset you may as well use a cheaper fabric and plastic/replaceable boning that's cheaper (and easier to dance in).
Fitting
This one haunts me in my sleep. If the lacing gap exceeds 3 or 4", it's too small. Your seams won't line up where they should on your body. It will probably be uncomfortable to wear (and perform in!)
Also worth noting; figure-modifying corsets (usually with a busk closure) are sized 2" less than your natural waistline. So if you have a 26" waist you would buy a 24" corset. If the gap is more than 3 or 4" your corset doesn't fit you. The seams are designed to lay against the body in specific places, accounting for the curves of your front, back, and sides. If the corsets back is creeping around to your sides, it's not giving you the proper support it's designed to.
Finally
Know what you're paying for. If you're buying a professional-made corset you want to make sure you are getting your monies worth. If you are making your own costume for floorshow it's fine to use affordable options so long as you understand the corners you are cutting.
