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.
Conversely, you could always get an ANSI terminal 102-key, and have a full layout for the keycaps too. If the keyboard costs less than an entire keyset from Unicomp. :)
Especially the 1395162. (Which is a 101 terminal board...)
Show Image
(http://i.imgur.com/2x446mW.jpg)
Yes, quite true. And then if I find a used board that's working, that's a donor too, except that it works so anything I take off that board will need another donor, lol. I guess this is how we keep buying things!