Garage sales. I have found more good wheels at garage sales, than anywhere else. Anything from Shimano 105 hub on up. My best flip ever was a set of Zipp carbon wheels, complete with tubular tires, skewers, cassette. Also got a set of Mavic Cosmic Elites at the same garage sale (with tires and tubes). The typical garage sale wheels are either left overs from when the owner upgraded, or a single wheel left over from a crash.

Found four wheels at the Goodwill Clearance center last week, two matching sets (but MTB wheels unfortunately). They sell stuff there $1.10 a pound. So I could not pass them up. One set Shimano STX with Mavic rims, the other set Shimano Parallax with Weinmann rims. Both with SS spokes. One will go on that Univega Alpina Pro I just picked up. One wheel was missing the QR.

Bought a Shimano 600 wheelset at a garage sale two weeks ago, complete with cassette, tires, and tubes.

The wheel I am trying to match right now is a Ultegra 6500 hub, unknown rim (I think it is Mavic). Not home right now, or I would take a closer look. One of the fronts I have has a Shimano Ultegra 6600 hub, with Mavic Open Pro rim (black). I also have a low spoke Shimano WH 6700 front (I think). Thinking about buying a black Mavic rim and relacing the 6500 rear hub. The 6600 hub doesn't match the 6600 for sure, so I am not jumping on that option.