I got the boards at amazon for $15 shipped for a pk of 100 (I have prime right now but you can get 2 packs and get free shipping). Or if you have a store that sells comic books, you can probably get them there cheaper. I think I read that most stores sell them for less than $10.
And to answer other questions people might have. These boards are fairly bendy but kind of like that cardboard that shirt boxes are made of. But a little bit stiffer. I could wrap 3 yards on them (the fattest bolts you see) but I wouldn't put any more on one.