Have you thought about using surveys on your website to track where your visitors are coming from and how they heard about you. You can see a great example of this at Cravenplan

Surveys are also a great way to discover what your customers think of the service you are offering and it is amazing the insight you can get on what products they were hoping to find.

Using this information you can target your advertising to the right market and can make sure that you are stocking products that these visitors want to buy, this will increase conversion rates and you can make more money per visitor.