No prob - a most of them are from the advertisers/brands themselves through the big affiliate networks. Almost all of them are on Commission Junction. The few left over are on Linkshare and even less on the others like Impact Radius, etc. Google shut down their affiliate network a few months ago and almost all of the brands went to Commission Junction and Linkshare.

For coupon codes you can't find from the retailers themselves, you can take a look at competitor websites.