Apart from Yard sales/garage sales, and film collectors, there aren’t a lot of options.
Projectors are expensive, so you are better off buying one from someone who services them and get one that is in tip top condition, that has new belts and has been properly serviced and lubricated etc. otherwise you will end up with nothing but trouble.

There are so many projectors out there it is hard to recommend particular models. You want one that has easy access to remove the gate and pressure plate, as you probably want to file those out a little to expose more of the film image area, and also ease of removing the shutter, as you will also want to remove it. Something that is known to be reliable and gentle on film is also a good idea.
The Bauer projectors are good for S8 and 16mm, the Hokushin 16mm projectors typically have an open film path and are easy to work on.
The main thing is to get one in great condition, or one that has been serviced with new belts and rubber parts replaced. It will cost hundreds, but the rest of the gear to catpure in hidef in realtime is going to set you back around $1000-$2000 or so in total, so it isn’t the expensive part of the build.