You are building the ad targeted site or you are looking for 3rd party targeting? Or do you mean, you want an IP-to-location lookup? Those dbs are available on plenty for free or a small fee(simple googling would find those).
Digital Element has the best data, but they are on the expensive end and their software isn't all that flexible in terms of deployment. Maxmind GeoIP is weaker in terms of accuracy but are far more flexible.
I've been using hostip.info, but have had mixed results - it's free. I suspect i'll be moving to a paid service in the future.