We get alot through the mail, from certain companies and also in the Sunday paper.
One tip- if you have a Bilo in your area they almost always have "double coupons" and any coupon 60 cent or under they will double. Sometimes if I play it right, and have a coupon for something they have a sale on I can get it for just a couple cents.
Also, though not for groceries, I love the site-
www.retailmenot.com Anytime I'm planning on ordering something from online I always go there to see if they have a coupon code for the place i'm ordering from.