Internet Archive: Free Download: Sound Recording and Reproduction (Sound on Film)
Apart from being a great little film about early sound technology, this is public domain and has a good few sections where the optical track crackles along nicely without any of that annoying speech, music or sound effects stuff we all insist on putting on films these days. With some judicious editing you could get a useable amount from it.
I think (and you should check) you could take the optical crackle off this without fear of copyright problems. I just watched the live stream which is a bit 'digital compression for the net' sounding but there are download options which might give you a more appropriately analoguey sound. Hi-fi lo-fi!
Rob