Zillions of Chinese ones on eBay some are quite good quality and range from $5 yes $5 to $90 but these expensive ones usually come with a riders backrest for Harleys or other makes etc but are easy to use and higher quality with velcro vinyl zipped bag attachments for the back.
The stock mounting system and flat bracket make virtually any pad usable as nearly all have a backplate of wood or hard plastic to screw into. The higher quality ones have threaded bosses/inserts already fitted in the pad's backboard but these are unlikely to align with the 4 stock holes.
I have some classy new R3Touring Longhaul pillion back-pad kits. These are intended for the round Touring sissy bar but use a 2 part back pad and 4 bolts with Sissy Bar Badge Embellisher, Blade, Adhesive A9930156 and Chrome steel back-plate so should be easy to mod to fit the R3Roadster version.
Longhaul Sissy Bar Back Rest Pad Kit A9708236
Longhaul Passenger Backrest Pad Kit - Limited Edition
I ordered from them. They canceled my order saying that they are no longer available. I think I'm screwed. Bought back rest for 300. Now can't find pad.