if it's a 26x2.5 it's the same as a bicycle. antique motorcycles used 26" tires. you can get them from Coker tire but they ain't cheap.
modern motorcycles measure the rim like was said above. the only crossover is the 16" tire which fits a 20" bicycle rim. no other mc tire sizes have been found to fit 24's or 26's.
servicycles are a rare breed and still go by bicycle tire standards. if you can find the tires, they'll fit your 26" rim.