The stuff I use is called "Dura-Lar," made by Grafix. It's technically not acetate, they call it the "acetate alternative," it's archival quality, will not yellow, is dimensionally stable. It comes in 9x12 pads, 25 sheets per pad, in three different thicknesses (.003, .004, .005). I bought mine at Hobby Lobby, but I also found this link:
http://www.colinharbut.com/art/Dura-Lar-Clear-Acetate-Alternative/drafting-architecture/acetate/