I started with Quilterscache's instructions and then some free online patterns. Then I moved to Carol Doak's patterns and on to Judy Niemeyer and Norah McMeeking. I do the actual paper piecing where I sew on the lines. Both with JN and NM, I bought the preprinted papers. For CD, I bought a book and found that I had a tough time with copying the patterns -- I thought all the blocks were the same size and they ended up different somehow...and honestly I've never gone back to figure out how to fit them together to finish.
However the preprinted ones can be pricey, so I try to find them on resale on ebay, etc for people who bought and never made!