You buy dozens of "Cruelty Free" animal traps and fill them with food that's just about to go bad. You place them in your yard and all around your house. A couple days later you go out to collect city rats.
The cleverest city rats would not fall for such a simple trap, but you feel like it will be easier to civilize the stupid ones, so this doesn't bother you much at all.
All told, after dumping out the dead ones, you have your hands on eleven reasonably healthy city rats, ready for their education.