Typically, you should join the the big affiliate networks; Linkshare, Commission Junction, Shareasale, Amazon and eBay. If you have decent traffic you may get some publisher support (you are what the networks would call a publisher, i.e., you are publishing their clients adds). The reality of each network and then program, is that your success will be in matching the offer to your audience and the experience you are offering. You might be inserting product data feeds and not getting a very high conversion, but since you have tons of traffic, you are doing well, or you may have a few really strong content pieces that you can work in links.
Rather then just looking at the networks, go out and hunt the right type of merchants. Just like you would when doing b2b sales research, you, or an intern, could go google those categories and see how many of the merchants have referral or affiliate programs. Then based upon what you see as a good fit, sign up where they have their programs, most merchants will have their program on a network, but more and more, folk are running private offers.
Drop me a line if you have any questions.