I would have expected to find them in, e.g., Wellingtons and Sunderlands postwar. In other words, anything continuing to use operational Frazer Nash turrets.

I would guess most 10 Group Lancasters had them when they were flown home after the war. Many were converted to non-bomber roles in the forties and the turrets and other equipment may have become redundant. A fair bit of Lanc stuff seems to survive over there, so perhaps Canada is your best bet?

That said, I found a n.o.s. F.N. relief valve in a junk shop a few years ago – you never know what will turn up! 🙂