I have had a lot of success using Papers For Foundation Piecing that is put out by That Patchwork Place. It comes in a pacakge of 100 sheets, and I paid around $9.00 for it. The printer prints perfectly on it, you can iron over it and the ink doesn't smudge and the paper doesn't wrinkle up, and it folds easily. The computer or typewriter paper works o.k, but it can be hard to remove.
Carol Doak's paper is also very good. And if anyone has any questions about paper piecing you should go to her website. She has a ton of information, free patterns, and a store where she sells the supplies. She also has an excellent DVD to learn how to paper piece. Her website is www.caroldoak.com.
Grace