How do I power a scavenged motor?
You need something that will provide 120V DC.
That could be as simple as a diode between it and the mains (in the US), but that could be a little dangerous...
Personally I'd have a 1:1 isolating transformer (or 2:1 if you're in the UK/europe) to separate it from the mains (pull apart a shaving socket for one of those) and then a rectifier circuit (multiple diodes).
Oh, and don't forget a fuse ;)
The motor will most probably have originally been wired up with 2 diodes like this:

Where the switch S1 selects between one of two diodes (middle position is off) to set the direction of the motor for forward (shred) or reverse (jam clear).