You might try 3M they make a bunch of different backup pads for buffers, grinders and random orbital sanders. Here is a link to their backup discs for grinders/buffers in the industrial and manufacturing section of their website.
3M grinder/buffer backup pads They have a listing for a 5 inch pad that is 1/2 inch thick and attaches with a 5/8-11 internal(female) thread and uses PSA discs, but they only sell them in packs of 5 or 10.
they also have a listing for a 5 inch pad that is 3/4 inch thick
You might cross reference their UPC codes or 3M codes at grainger etc... or google them and see if anyone sells them in singles. Hope this helps.
on a side note I have used a friends 7 inch buffer from that he got from harbor freight, it uses hookit style sanding discs that they also sell, to remove paint from body panels.
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=92623 you might try ordering a replacement backing pad from them and peel the hookit material off and file it down to 5 inch or 6 inch diameter, it uses a 5/8-11 female thread to attach to the arbor.
