I have used foam board from the dollar store which is also acid free and works well. I like it a lot - very firm and my fabric looks great. The down side it takes up more room and you have to cut it. The thing about cutting it is you can cut it to the size you want.
I recently purchased some comic book boards and while they are not as firm, they do take a lot less room in shelves so I think that is the direction I will be going except for large pieces of fabric that I will continue to use the dollar store boards for.
I do have to say that which ever board you choose it is a far better way to store fabric for all the reasons that other people have mentioned.