Just a quick explanation of the different kinds of boning out there.
What Not to Use:
If you go to a chain craft store, you are likely to only find cheap plastic boning, usually in the sew on variety (center) and plastic in casing (left). DON'T use these. They twist and buckle, and are bulky at best. Avoid them!
What to Use:
You want to use steel (not pictured) and spiral bones (right). Steel bones can be used in the side or back seams, where there is no curve. Spiral bones are used for the curved seams, as they will contour to fit the seams of a corset where a straight bone would buckle. In this instructable, I used spiral for all the seams because that is what I had on hand, but in theory most of my seams could have used steel bones, since only the front seams at the bust have much curve to them.
Where to Get it:
You can order bones and other corset supplies online at either the US based Corsetmaking.com
or UK based Vena Cava Design.
What Length to Buy:
You can cut your bones and cap them yourself, but if you already have your pattern you will save yourself a lot of trouble by purchasing boning to size. You want to buy bones around 3/4" shorter than your seam, otherwise it will create too much stress on the fabric and potentially break through after a few wearings (or one wearing, as I found out with my first corset!).