Bryant,
You have two options here, that I really see would be quickly to implement.
Option 1:
Google analytics has referral information, you can gather this information on a single page if you have it tracked within Google analytics. You can do a search about making sure your building a report correctly or how to navigate to get this data.
Option 2:
Create a javascript snippet on the landing page to get the referrer and pump it into a hidden field that is then pushed to Marketo when the person submits the form. The problem with this is that you might not get all referral data, and lets use this scenario. One website provides 100 clicks and only 20 convert and the other provides 300 clicks but you get 20 converts. You wouldn't be able to segment the better conversion of the other referrer.
Doing both can get you the full circle of Referrers to clicks. but having different different systems my not give you perfect numbers, but a good estimate of the overall effectiveness of each ad placement. If you're using 3rd party for hosting these ads, most of them have at least the click data.