Homebrew Exchange now exchanges CO2 tanks.
Here is a list of some places where you can fill CO2 tanks for your kegging setup. This list will be a work in progress. Please add your suggestions on where to fill CO2 tanks.
These places require you to turn in your existing tank and get a different one that is already filled.
Fill your own tank
Other Places to Try
Not sure if these ones will fill CO2. Will update after calling them.