There are many places to sell advertising. Text-Link-Ads.com is one of the oldest link brokers. There are other sites like blogsvertise.com that are market leaders in the text link selling space.
In order to help advertisers find you be sure to rank for phrases like, Niche + Blog, get on to lists of blogs as well as get some awards and be listed there. Then put up a link on your site to an Advertising section that lists your contact information for privately discussing advertising arrangements.
Selling text links violates Google's Webmaster Guidelines so if you want to keep your rankings and your PageRank then should consider the risks involved and either don't do it or commit to being discrete.