Also, are you musically and technologically inclined? Do you know how to, or could you figure out how to MIDI sequence?
All of the work of the old dead classical composers is public domain. What isn't public domain is any recording of it. But if you know how to sequence, it is (relatively) quite easy to make a classical piano composition fit what you want. And that way, the recording is yours.
I would limit it to piano, because piano is easy to make sound "real", as opposed to, say, a full symphony, or even just a few stringed instruments. Unless you're a total pro, any instrument except for piano is going to sound fake and synthesized.
The following is an example of how I used MIDI sequencing to adapt a classical piece to fit what I needed:
The original (not public domain):
https://www.youtube.com/watch?v=mXGSfJn3nKQ
My MIDI-sequenced adaptation:
https://www.youtube.com/watch?v=Mtn8-nbQL5g