Forum Moderators: DixonJones
Then, when a purchase is successful, keep the session referrer information with the order in your database (or simpler, have your site send you an e-mail with this info).
Of course, that's assuming you know how to code. Google Adwords has a conversion tracker that lets you track which AD a user bought from. That's handy, but not universal.
What you really want to do is run a search on "conversion tracking". That'll return several services you might be interested in.