I got my original set of boards from amazon.com but I had to pay for shipping and they are quite heavy.
Then I decided to hit the local comic book shop. Learned something new when I found one quite near here. And learned that they cost a lot!
But the boards were $9.95 for 100 and no shipping.
glen: check to see if you have a local comic book shop, I recommend buying local when you can. If not, then check out amazon.com for comic book boards