I buy it at Office Depot, but I'm sure the other stores have it as well. i don't know how big you need your sheets, but I usually get the 11x17 pad of 50 pages, and then cut the pieces in half (using the cutter at the copy center in the store) so I have 100 sheets. I think the pad is $16
If you're not putting it through the copier, the tracing paper is cheaper.
Nancy's Notions used to have a 100 page pack for $10 that I would buy at shows - it was always the best deal around.
ah, I see they still sell them online:
http://www.nancysnotions.com/product...ation+paper.do
That's the best deal...but then there's shipping and waiting. ;-)