The ad agencies have really got involved with this in the last few years (because of the kudos attacheed to uncovering an unknown artist and seeing them get a hit off the back of the ad.... though that doesn't stop them all wanting to use famous artists too) and I would imagine the larger agenncies have Music Specialists of some description? One company that's being doing this for a long time is JWM - Jeff Wayne Music - yes, *that* Jeff Wayne.