You're going to have a difficult time locating an aftermarket custom seat to fit your R65. The R45/65 series Airheads were not produced in great numbers over many years as were the Type 247 Airheads that were blessed with aftermarket options.
You did not mention what style seat you're searching for -cafe, comfy tush, ect- but a quality upholstery shop could fabricated something you might design by using your OEM seat pan, new foam and cover to your specs.