I think you're going to be looking at a prop RPM of 1000-1500RPM top whack if that prop has any decent amount of pitch, and really that is going to mean a gearbox or belt drive of some description.
MFA/Como do a comprehensive range, and they're reasonably priced too, e.g.-http://www.mfacomodrills.com/gearboxes/986d_series.html
Another possibility, is to check out breakers yards. Motors from radiator fan drives, heater blowers etc. can be suitable to run direct drive, as they're wound for very low RPM and high torque, but it can be a bit of a gamble, plus good breakers yards are getting a bit thin on the ground nowadays.