Atari 520ST (probably STF or STFM) in Sanford, FL. Perhaps a bit late to bid, depending on whether you read the forum when this is posted.
Atari 520STE 4MB RAM, 540MB HDD located in Meadville. I believe the STE model is completely backwards compatible with the older ST series, but adds new features. It is also less common to find. This one even comes in its original box which despite being tatty should add to the value if you manage to get it cheaply.
Otherwise, I would think you may post an ad on some freecycle list or likewise. Perhaps there is a usergroup nearby? I don't know about how common the Atari ST line was in the USA, but over here they're reasonably common and usually don't go for much money unless in a very good condition or something extra included.
You should also look for 1040's. It has twice the memory than a 520 but otherwise should be the same. I would expect the cost for those two being the same.
Atari 1040STF with two Megafile hard disks located in Daly City, CA. However those are mostly untested, and STF means you need to have a monitor cable since it won't connect to a TV (no internal RF modulator).
Atari 1040STF with SCSI located in Vancouver, WA. The seller adds a lot of nonsense sales pitch, but I believe all the technical details he mention are standard features, no particular upgrades. At least it is said to be tested working, but again STF means you need a monitor and/or a cable.
Atari 1040ST of unknown variation, located in Victoria, BC, Canada but will ship to the USA for $28.75.
Oh well, those were the ones I found right now without going further abroad than Canada. You need to search for combinations of Atari, ST, 520, 520ST, 1040, 1040ST and perhaps those STF, STFM, STE variations as well to spot all. Good luck.