Hi,
In order to track the source of payments you would need to redirect from paypal to a url which has 'something' in it which can be used to attribute the transaction to the specific source.
So when the visitor comes in to your site, we will capture the source information and hold that in a session cookie. When this guy goes to the checkout page, we will read the session cookie and change the return url from paypal. When paypal returns the guy to your site, we will take the parameter we sent and stick in into a custom event/variable so you will know exactly which source this guy came from.
This kind of work will require advanced javascript skills and knowledge of Google Analytics which I can help you with. I am new on this site but have a few happy customers on a different site which I am happy to share with you. Also happy to have a skype chat to explain how I would do this.
Thanks,
Dino