I'm looking for a plugin

I have found plenty of plugins to record all of the referrers to my site. But what I need to know is when a purchase is made how can I match where that visitor came from? Whoever makes a purchase on my site, I need to know how this visitor came to my site.

With the plug ins I have now, I’m not sure how I can easily connect the customer to the visitor. I hope I have made my question clear.

If anyone knows of a plug in or any way that I can record this information, I would love to know. Please help me out if you have a suggestion.

Thanks in advance.
Nickpl