Yeah a media agency is what you're after. If you're based in London there will be absolutely loads.
We have an actual media department that deals with all that stuff. Basically, a lot of the time they are just ringing up publications and asking to speak to someone about placing an ad in that publication. They will ring up new ones everyday. If you wanted to cut out the middle man you could try doing this yourself but it's quite time consuming and you really need to know what you're doing.