You can use any mission for this, but I used Murder of Crows in the Mardi Gras level. Kill the would-be assassins as you would normally so you can finish the level. Then simply kill various people and keep track until you hit 47. Then Escape the level.
I recommend saving immediately before exiting so you can see what how many kills the game tallied. If you end up short, use the "restart" option (hit Y) and then when the level starts back at the beginning, Load Game and pick that save file.
For those who get confused as to why they can't just pick off members of the crowd and hit 47, here's why: You may notice your target reticle doesn't turn red when wandering through the crowd. Another sign is that the crowd doesn't really react or have particular AI. It just walks around, whereas the other characters run, scream, hide, etc.
I consider the hundreds of people just "filler." They don't count as characters, and that's why if you mow down the crowd with an assault rifle, you don't get a killcount through the roof.
If you need more "legit" kills, use cops, the birdmen guards, chefs/bartenders, and go into the buildings for people dressed in the various costumes (rock, etc). Your targeting reticle will turn red when you're pointing at a "real" character.
Hope this helps. Enjoy!