At the end of Thugtown, after the propeller chase, is the area with the TNT and lots of zombies. You will get your first rocket launcher here. If you can get enough zombies chasing after you (and there are lot here), fire off a shot and hope 5 get caught in the explosion.
If you're looking to grind out some quick achievements, this is also a great place for farming a lot of zombie kills, chainsaw kills, and cannolis since there are 3 of them in this little area. I got all of these in this area.
If you set off the TNT and wait too long, you'll get a checkpoint, and the big zombies will come out. If you restart the checkpoint after getting a fair number of kills, you can really rack up the zombie kills in a hurry.