If anyone is in the Vancouver WA area,
I Like Comics
2101 East 4th Plain Boulevard (Just off interstate 5, near the VA hospital and Clark College, and they have off-street parking)
Vancouver, Washington 98661 sells comic book boards in several sizes.
I just got 100- 6 3/4" x 10 1/2" for only $7, and 100- 8 1/2 x 11 boards for just over $8, plus tax, of course. (I guess I could have gone to Portland and not paid the tax, but would have spent that in gas to go there.)
I'll cut the larger boards in half to wrap my smaller fabric pieces. I ordered the last ones from amazon.com and I paid quite a bit more for them, plus shipping, so I was happy to find this shop, just a few minutes from my house.