Most people I know use what ever cardstock they can find/afford. I tend to use cardstock from WalMart & Joanne's but have also seen people use playing cards, a painter used paper and for a recycle themed set another gal used cardboard, then there was that one girl who mentioned poasterboard. So it is kinda all over as to what people use. . .
Some gals have even made fabric. . .quilted ATCs. . .