What type of motor do I need for my attic fan?
My dad had to replace one of these at his house a while back and it wasn't much cheaper to buy the motor separately than it was to just replace the whole whole unit with one from a big box store.   If you can get a deal on the motor it may be cheaper though. 

If you do find a motor, the thermal protection is important.  If you have a heating and air conditioning place nearby, you may see if they have some similar ones in stock. 

If you type the model number into google (dow motor model d-xin-psc-6p1), you can find several of the exact same motor with different amps. I'm not sure how important the amps are, but it seems like the higher amp units have bigger blades.

Lowes has a replacement that has higher amps but is obviously made for a similar fan. 

A lot of these seem to be made by 'Air Vent Inc.', Amazon has some too.