I saw scripts doing this. They allow you create affiliate accounts and register where the sales come from. I do not know a lot about them because I have not used them, but I would like to suggest you to search Google for "affiliate program scripts" or similar search. I found many entries with "affiliate program scripts".
Do some keyword research using google hxxp://www.freeadwordstool.com and select a niche you are interested in. Then go to find the affiliate programs in clickbank or amazon
you can go to hotscripts dot com to search for an affiliate scripts. Or you can go here affiliate-scripts dot com