A list of current tank mods and where you can get them. Please feel free to let me know if I am missing any.
Clouds Of Vapor. I have personally used this tank and found it very solid. I did have some leaking issue when I placed it on it's side. Occasionally I would find that the CE2's gave off a funny flavor.
http://www.vaporescence.com/. Comes in a variety sizes and styles. I must admit the stainless steel model looks pretty awesome.
Bombay Vaping. 4ml ce2 tank that looks decent. Have not heard much about these yet.