Japanese maker LS made a 50's-60's (US school bus like) Isuzu Bonnet, in 1/32, also boxed by ARII, a rare & really nice kit.
Heller as mention earlier, did make a nice 30's-50's Renault TN6 city bus in 1/24, was repop in the mid 90's.
Many, many, resin kits are available, including double deckers, but in much smaller scale (HO).