Ask HN: Closing the coupon redemption loop... Where to start?

I've got some ideas on a mobile coupon/rewards app and I wanted to know if there's a company out there with a big database of coupons for CPG (consumer product goods) like groceries or for specialty retail (e.g. $10 off XYZ headphones) at Best Buy for example.

So far, I know of coupons.com & catalina but what are some other sources out there for potential integration that can be redeemed at checkout?