I've recently got a 12ft john boat, and i need a motor for it, good for a river. that isn't too deep. Where could i find a motor, and what size?

my freind has a 11 ft john boat with a 3 hp tanaka outboard that does good in creeks and rivers.

Thanks for the input, also though, i need some horses to wear i can go up and down river quickly.....but i don't want the motor to be so big as to flip the boat over.