You can get a spray, but I've never used it so can't tell you.
You can get the spot on from P@H
(here) and you just have to put a drop or two (instructions on the back will tell you how much, by weight etc) between their shoulders and then brush it back, avoiding the head.
I have 12 rats, so I have
this one as it works out much better value for money since I have so many to treat.