They are all over. It just depends on who you ask. Being in Phoenix
area, I don't know all of the farming requirements, but I know up here,
a 5hp motor is pretty common for an auger motor. A 7.5 would be an
average size roller mill motor.

If he needs 3450 rpm motors, find a dairy industry. Nearly all of
their pumps and vacuum motors are run at 3450. Given the area he is
in, I know there is a lot of irrigation done, so I would expect there
are some very large motors running some of that. If it was me, I would
hit up some of the local ag industry suppliers.

A new one would be expensive, but if this project is as noble as he
presents, it wouldn't seem to be that hard to get a few businesses to
donate one.