There are many places like close to Thais Temple, Edron in the streets, in the General Murius spawn at Mintwallin..
On Rapid Respawn they are easy to complete since they require only 25 kills, so your easiest spot is close to Thais temple or at Lubo's house (where you get the Citizen Backpack addon). Just stick there and wait for it to respawn and it will be fast.
As for Cats (if you don't have), Carlin has 2 if I'm not wrong.