I belong to a Mom's group and we just had a meeting on this issue-coupons and saving $$$.
Here's what I learned:
Thegrocerygame.com- costs $15 for 8 weeks- gives you the lowest price on an item in your local stores.
coupondivas.com
saveathomemommy.com
Totallytarget.com
Target.com- for coupons
familyfriendlyfrugality.com
couponmom.com
You can always go to the specific company for their own coupons
prudenthomemaker.com
Michaels' and Jo-Ann's have their sign up's in the store for their mailing list by email or by mail. There may be a way to sign up online as well.
Also, getting your local Sunday newspaper is my best tip on getting great coupons.
Hope this helps!