They come up on Ebay, particularly from the U.S.
Expect to pay a fair bit though and then is it genuine and will it work?
I ended up putting a brand new one in my 70's P, figuring that at least it would be reliable. I will include the original when I sell though.
If you have the pots they can sometimes be repaired or at least adjusted to get them working again. If you are not confident in going down the DIY route, get in touch with Bass Doc he's the man