Look for a "Habitat ReStore" , or any type of place that re-sells used, dinged, or builder reject building supplies. All of the wall cabinets in my old garage, all of the base, wall cabinets, and doors in my new shop, and the base cabinets and countertops in the garage I picked up cheap from Habitat Restore.
I just take some measurements of the space I need cabinets for, then spend some time there with a tape measure and pick and choose what I need. I've had to do a little modification and painting, but there's tons of doors, drawers, hinges, etc to mix and match.
Sometimes you can find cabinets cheap or free on Craigslist, but you've got to be quick, you're stuck with what they've got, and half the time they want to remove them from the walls for them.