I know using Google adverts will probably be the easiest way but I dont know if I might be better off approaching some companies that are in the business that I am aiming my apps at
Personally I'd want to go with the latter but it would probably involve alot more work. alot of it depends on whether you're building a straight portal or a community. If it's a straight portal your punters aren't going to care less about the nature of the adverts. If you're building a community then more specific advertising is definitely the way to go.

A freind of mine runs a dungeons and dragons web site (yes, he's a geek... but he's a geek whose currently making lots of money for doing naff all work so you've got to hand it to him). It's very much a community affair and when he experimented with generic advertise there was a near revolt. Keeping his advertising niche means that the punters are actually quite pleased by its presence.