Here is a great site to get sites by category or keyword , many users report MFA sites here http://www.adsblacklist.com/
The most obvious cheap ads that will appear in adsense is ads that advertised by big boys such as wallmart.com, target.com, amazon.com. But using adsblacklist.com is the fastest method to get low performancing ads on your blog because they will scan your blog and find the ads and list it out for you. I was using the list provided by adsblacklist.com 2 months ago and it pushed up my lowest CPC from 0.1-0.2 to around 0.35. Well, different site different case. So it might be different at your side.