First always check a manufacturers website, then check their facebook pages, you can almost always find a coupon your looking for. Also, if you use a product often enough contact the manufacturer and ask them if they have any coupons they have to mail. Sometimes you get some great coupons, sometimes you get turned down.
Coupon Network by Catalina. you do have to sign up in order to print
Target.com You will find both manufacturer coupons and Target store coupons here.
Boxtops4education.com you will have to sign up in order to print
I'll be adding more as I go along. These here are a few of the majors for printable coupons.