Alot depends on the size of the agencies you are going after. Many (unfortunately) no longer have a dedicated art buyer role.
I love the A Photo Editor blog that Ken linked. Read through that and do some research on your specific targets (if going after specific local agencies). Many people also like the various list subscriptions (Adbase, Agency Access) - which will offer appropriate contacts for given target agencies.
Also, heading to the library to go through the red book can be helpful.
FWIW, in lieu of an art buyer, I usually aim for art directors, since the vast majority of creative directors seem to come from the writing side. Also having worked as an AD, its fun to get well made, well thought out promos.