Of course you could just connect a "stand-alone" PIR to a 3-pin socket
(indoors), then plug in a standard plug-in transformer to give 5v or
whatever when the PIR triggers. If you are confident working with low
voltages you can build whatever you want to run off this low voltage
output.

Do make sure that you properly label the socket though - you don't want
the next door neighbour (who pops in to feed the cat/dog/fish while you
are on holiday) plugging a hoover in!!