They appear in chains around the whole city, and aren't common. Killing them all in 5 seconds will grant Claude the Elvis Has Left the Building! bonus.
GTA 2: The Movie
In the penultimate scene, Claude Speed brings a group of Russian mobsters to a bar frequented by Rednecks and Yakuza enforcers, and Elvis appears to be performing live on stage. It is unclear if this is meant to be the real Elvis or an impersonator.
GTA San Andreas
They are very commonly spotted around Las Venturas, however they are just regular pedestrians and Carl will get nothing for killing them, except some cash that they may drop.