ACtually, it's quite easy to find 16mm and 35mm release prints in both scope and flat format (spherical and anamorphic projection). You have to be careful of prices because as always some people are stupid.

There are many choices on Ebay. I did a quick search and some of the old websites I use to frequent are gone, new sites pop up all the time.

There are many more classic films to be had in 16mm. There are many uninformed folks who say that you can not get 35mm release prints no matter what - they are just wrong.
You can, if you want, even obtain 70mm prints, but THOSE projectors are a) hard to come by, and 2) take much electricity and cooling to run.

Good luck! There are about 30 preview trailers at the local second-hand store right now. The theatre I grew up with would routeenly throw away into the dumpster, 35mm prints or preview material that was "past its' life" .