What is Trade Doubler?

I would say, analyse your market, the best affiliate sites will always be ones that are related in some way to your business as they are most likely to garner click throughs. Generally Affliate Marketing companies should be able to give you a good lists of available sites in your sector. You can always go on to Google AdWords / AdSense set up an account and use their tools to see what sites you could place your ads on and what potential traffic that would garner.