Hi Smudger. As Sylvia says your local agricultural merchant will have them, about £30. I leave apple in the end or drop chicken feed in from the top. Just be patient, they will go in eventually. Alternatively bury a Fenn MK4 in the rat run and cover it over with soil and the a cover over the whole lot so the rat runs down a tunnel. Nasty traps though. Break your finger easily! The little rats and mice go in and escape through the holes -the big ones then think it's safe. Same with the Fenn -little rat is too light to set it off so they establish a run over it and the big ones follow -Bang!!!
I'm suprised rat catching isn't more popular as a recreational activity. A real challenge and exciting when you go down to check the traps in the morning. Had 7 in one trap once! Had 12 in the dustbin waiting for collection once as well.