I'm woking on a startup where we will put coupon codes on our website and in return we will get 3% of the net revenue of that sale. My question is how could I get vendors on this website? I don't (really) want to buy coupon lists from those websites. How do I get vendor's coupons on my website? Thanks in advance!
I don't think there is any automated way to successfully do this. You'll probably need to sell the old fashion way. Prospect potential customers, try to get them to listen to the benefits of your service and sell them.
I would start with some verticals. Target businesses in a specific industry like apparel... then use Google to find prospects.
Tricky part about your idea is that you are going to have to know how much they purchased on the other website right? So whoever you get as clients will need to have some affiliate or tracking system to track sales from your site, and to ensure you get your kick-back?