Google the product you want, and the best affiliates will come up on top.

Find the squeeze pages for as many affiliates as you can.

Use the link: operator in google to see the relative number of inbound links. Top affiliates will have more.

Check Alexa rankings for their squeeze page as well.

Edit: Ok, ok I know I am always telling everyone that you cannot depend on the link: operator to tell you the number of links. It only gives you a sample of backlinks. However in this case, we are comparing sites so the one with the biggest sample is still likely the best at IM.

As Pete says, the only tool you use is Google and your own brain power.

There are no tools out there to do the job you want.

Agreed, Google and your own brain power are the only ones, nothing in life is ever perfect