Look at the parts used by Pololu on the h-bridge boards.
Most of the "continuous" ratings assume mega-heatsinking - watch out for that. Find the Rds(on) values for the bridge andcalculate the heat dissipation (remember two devices are on at once).Alas there seems to be a big gulf between the performance of discrete MOSFETs and ones integrated into bridge ICs. Lots ofbridge ICs quote 0.2 to 0.3ohms or so, discrete SOIC/Dpak/TO220 MOSFETs of 0.01ohm or better are easy to find. This ispresumably because discrete FETs are vertical current flow.