I agree that you will be very lucky to find anything decent in a thrift store. 5 years ago, maybe. 15 years ago, definitely.
If by "parts" you mean anything but key caps, terminal boards usually sell much cheaper, but usually lack exactly the keys that you are probably looking for.
As for cases, mats, springs, barrel plates, etc, a single carcass will yield lots of parts for cheaper than ordering from Unicomp.