Commission Junction is great with the coupons, promotions, and promo codes. Check out CJ at commissionjunction.com and there's also an additional forum here's that has some great info on them.
There are many merchants in CJ, Linkshare, and Shareasale that provide coupon codes. You have to join there and then apply to the merchants you are interested in. Then ask them for a coupon code. Make sure you keep your coupon codes current. There is nothing worse for a consumer than to go to a merchant with a coupon code and it be expired or not working properly.