Joe, if you have some frames, that are correct in size, and are of a more solid construction, but came with plain cardboard backings, it is certainly MUCH cheaper and cost effective to go and purchase a sheet of acid-free foam core and cut it to the size of your frame.
Foam core is easy to cut, with a sharp, exacto-type blade and a straight edge.
That way, you're able to use the frames (and if they already have UV-plexi, that's another plus), rather than buying new, all the way around.
Again, just another option to consider.