www.oewarehouse.com or
www.oehq.com. Get AC Delco motors. There are two part numbers. Get the part number that does not have the mounting brackets and requires you to reuse your existing mounting brackets (with nuts/bolts.) The motors with pre-mounted brackets did not last as long for me.
One of these two suppliers has free shipping over $25.
Beware - most of my power window problems were due to bad wire connectors/harnesses, not motors or switches. I would check those first.