What is the best way of getting 125 X 125 ads on your site?

I see that pretty much all websites that are doing well, have 125X125 ads, and I’ve also read that they give good revenues. However I see many people sell them directly or use a website called buysellads.com
Does anybody know what is the best way of implementing 125 by 125 ads on a website?
Thanks in advance.

See original post by elperrillo