1. Free- Search for penny auction groups, the open ones, on Facebook, Yahoo, Google, and Bing. Make sure to obey the group rules, participate in the conversation, and recommend your penny auction site. Advertise on Classifieds, and any where that accepts ads.
2. Pay - You don't have to pay much to reach a lot of people. I went to a site that only cost $5. (Fiverr). I found so many options for advertising.