Some places to check online:
Electric Trading Co
MPJA
Goldmine-Elec
Quickar Electronics
All Electronics Corp
American Science & Surplus
Alltronics
Herbach & Rademan
Surplus Center
American Science and Surplus has some (IMO) really good AC Papst/ebm impellers for $12 or so which I'd use in an external box - use the impeller to pressurize the box with the rad(s) in it and let the air flow out through the rads. You'd have to mount the backside of the motor to a plate and mount that so that the impeller lined up with a hole in your case, so not as easy as "strap to a heater core", but maybe similar work if you were thinking of building a shroud or two...