First of all, vellum makes the paper piecing process much easier and the pattern preparation much faster. When I first developed my method of paper piecing I used regular copy paper because I wanted to eliminate the need to purchase special papers. However, in the early years I experimented with all different kinds of paper and discovered 17lb vellum. It was perfect for the process. And when I taught live classes I supplied the vellum patterns as part of the class materials.
Then came virtual classes and you needed to print your own patterns. Copy paper is easily available and almost everyone has copy paper. It works, but there's more pattern prep because copy paper isn't smooth and isn't translucent. For most of my patterns, copy paper is just fine because the patterns don't have lots of segments. "Not Your Daddy's Log Cabin" blocks have twenty-two segments and regular copy paper doesn't quite cut it. It's usable, but not ideal. Thus, the need for vellum.
You can look for your own 17# vellum, but the links I've supplied are brands that I've used before and I can definitely say that they are my favorites. If you choose to look for your own, make sure it's 17#(lb), 8.5" x 11" and feeds through your printer smoothly. Also, if you've never taken one of my classes before, you'll just have to trust that whichever quantity you order, one sheet of paper goes a long way.
So the bottom line is this...get the vellum. It goes through the printer the same as copy paper You'll find it changes everything and streamlines the process, thus eliminating time. Just click on the image for a direct link to order100 sheets, 50 sheets or 30 sheets. The 100 sheet option is the most economical, and order early to make sure you have your vellum in time for class.