Honestly, I bought mine at Walmart and they were "moving man" discs ~ located in the hardware section! they work just find for my STS experience! Plus they are cheap and some come multiple sets in a box!
"Gliding Discs" are a specific brand. They are good, and not very expensive.
Valslides are another option, but rather pricey. BUT, they are almost exactly the same as these: http://www.amazon.com/Waxman-470409...f=sr_1_14?ie=UTF8&s=hi&qid=1260158179&sr=1-14 (the only differences: Valslides are green, and they indent slightly on the sides).
Gliding discs are a bit better for moves where you need to use your heel as a brake by keeping it slightly off the slider (like side-to-side slides). Valslides are more comfortable and easier to use when doing push-ups.
A cheaper route would be to use paper plates on a carpeted floor, or rags on a wood floor.