If you go to the vintage machine shop page, there are dozens. Just type in 15-91 with the dash.
I have one, in a cabinet. Which is what I would recommend because she is HEAVY. She will also sew through anything you can get under the presser foot. The 15-91 is gear driven and all metal. She is the workhorse of domestic Singers.
I paid $60. for mine, in a cabinet, and considered it a steal. I see them occasionally in this area on Craigslist for anywhere from 50-150. The are really easy to use, easy to oil, and make a great stitch. I think if you can get it for under $75, you would be doing really well, and wouldn't regret it.