There are thousands of merchants who offer coupons out there through a combination of dozens of major affiliate networks and hundreds more private ones. CJ, Pepperjam, Linkshare, Google are among the top names.
However before you jump head in I suggest you really think about this. Coupon sites are not new and not basic... this is a mature market with defined leaders who have huge volume from sophisticated marketing that allows them to secure far better terms than you will get through generic network offers, who work directly with partners, who know the major seasonal drivers and who are looking to keep their business growing too.
You need differentiators. Something that makes your site in a class above these sites or to the side of it... I believe just about any market can be disrupted with the right vision but if you're not even able to find the affiliate players I'd suggest you seriously think about what you expect to accomplish and if the answer is anything more than a small site that makes a few bucks, you'll need to really dig to innovate.