I ended up gifted one, but when I started looking I put a wanted ad in the local nickle or trader paper. I indicated a Singer (so I could easily get needles and bobbins or other parts). And said I wanted it for actual use, hoping that would weed out the out in the bushes boat anchor responses. I got a good number of calls and asking prices were in the 30-100 dollar range. I also knew I wanted a standard bobbin, not shuttle, but didn't say that, most folk haven't a clue, why confuse the issue.
Good luck with the search! Most belt driven (via motor mount) Singers can be converted to treadle, so you have a huge variety to look for and aren't limited to just straight stitch, but can get ziz zag and then drop it into a Singer treadle stand. Let us know how the search goes!