The Procter & Gamble site also has coupons now: pgeveryday.com. These may be the same or similar to coupons in the monthly P&G newspaper insert. The Kellogg's website has lots of good coupons: kelloggsfamilyrewards.com. You will need to register with some of the manufacturer sites, like Kelloggs, to get the coupons.
If you have a smartphone, there are a number of smartphone apps, like Target Cartwheel, that save you the trouble of clipping or printing. There are also sites like SavingStar, where you register your supermarket loyalty card(s), select the offers you want, then when you buy the item the savings go to an account that you can periodically empty via bank transfer, or PayPal, or Amazon gift card, etc.
If you're looking for a specific coupon, check the manufacturer's Facebook for "offers." Sometimes, you will be lucky.