Ok nobody answered so... what bearings are you talking about? The bushings in the housing or the bearings on the axles? The axle bearings should be standard sealed nongreasable insert bearings. The big RTS uses 1" i'm not sure about the size on the smaller RTS frame. The bushings in the diff housing are available but you may have to search. The parts breakdown for the 159 shows 1" id x 1.125od x 2" long.
http://www.odref.com/peerless/differentials/100-SERIES/100-159.pdf
Outdoor Distributors - Lawn Mower Parts!
http://www.odref.com/peerless/differentials/100-SERIES/100-159.pdf
Outdoor Distributors - Lawn Mower Parts!