Well, I bought an 80's Peugeot with Simplex derailleur for $50 and though I had it made. (Even did a little happy dance)

Just walked back from my second ride having had the rear derailleur snap in half.
Where should I go to replace?
What compatibility issues are there?
Are there used original parts available or would I be better off getting something new or newish?

Please help. I am feeling very depressed and thinking about taking up bowling.

Thanks