Where would I find a muffler?
 

I am looking for a NOS or a good repop for a 69FB Sprint muffler.

Any ideas?

I may have one
unless your car is a show piece I would convert to duals
Use 400 exhaust from over the axle pipes ,cross flow muffler,tail pipes,hangers.
have a good shop bend a couple of head pipes to meet the duals.
You can add the factory style resonators if you want it a little quieter.