In most areas the local paper has weekly coupon insert including Smart Source (SS), Red Plum (RP), P&G (monthly), and occasionally General Mills (GM).
You may also obtain coupons online:
Coupons.com
SmartSource.com
RedPlum.com
MamboSprouts.com
Coupons are generally different online and in the inserts so it is good to get a paper and print coupons from the above links!