I use those same comic boards from Amazon. But I only use them for lengths of fabric 2 yards or less. They won't stand up on their own, but when they're lined up on a shelf with others next to them or with some sort of "bookend," they do fine. If you want more weight, just use two of them but you can't put too much fabric on even then. They're still pretty cost effective.
For lengths more than 2 yards, I fold them with a ruler then fold in half and store flat on a shelf. That way I know I have at least 2 yards of fabric.