Google, Yahoo, MSN, Adbrite, Fastclick, Value Click Media, directly from related websites. Those are the ones I can think if in a few seconds.
There are many place you can buy it but be more specific such as you want to advertise what kind of site. There is not make sense if you advertise your car product at animal related site. You can find it at DP advertising section.
You can buy banner/text link advertisement through any webmasters forum. Just go to Marketplace section and post your offer/request.
You want to have your advertising as TARGETED as possible so choose sites (blogs, forums, directory sections, etc.. ) that deal with your product/service very very closely.
I would go for one of the top 3. if your budget is small you could try Adbrite, although the traffic doesn't convert as well