I was able to go to a Comic Book store and see the various boards - can't remember what I bought but was assured they were what collectors use in their "Collectable" comic books. Mostly I bought the standard 7in. x 10in. - there is a slightly larger size I got by accident and just used them for NFL & MLB fabrics since they are 60in. wide.
It's been a couple years now and the fabrics are all fine. I do keep my fabric in bins (the ones that are made for files I got a Joann's) and being in So. Cal. I don't have an issue with moisture.
I also cut a lot in half ( 3.5 x 5) for fat quarters and pieces.
If your live near a large mall they may have a comic book store - I believe ours is called Comics & Stuff. I know it was under $10.00 for 100 board. I bought 300 to begin with and the guy asked me what I was going to do with them and was totally surprised when I told him. He thought I had inherited a large box of comics.