Fabric stores will give you their empty bolt cardboards. I'm not sure if they are acid free.
You can buy comic book boards on ebay in lg quantities that are acid free & right size for wrapping fabric.
Some mentioned on this site, buying thick poster board or thin foam insulation sheets & cutting to fit your size preference.
Another way to wrap without a board, takes less space, no cost, is to do this:
Fold fabric selvege to selvege. Wrap piece of fabric evenly around a ruler, I use 6 x 24 ruler I use when cutting quilt strips.
Slip out the ruler.
I fold the fabric in half or thirds at this point; fits just right on my shelves.
Use smaller ruler for FQs.
Occasionally open fold & fold opposite way to prevent constant crease in 1 spot.
Best to cover shelves (I use old sheet) so sun doesn't fade fabric at the fold.
You should get many suggestions - its a frequently discussed topic here.
Also try the search at top of board & look for how to fold fabric.
Here's a tutorial on folding using your ruler:
http://turningturning.com/tutorial-folding-fabric