I did a bit of googling and found a couple resources. If you know of any other place to get cylinders recertified, post them here and I will update the original post with the links.
Learn how to recertify your propane tank with AmeriGas including how often you should recertify and how to find out if your tank is due for recertification.
ASG is a DOT UE and Hydrostatic Testing Facility. We re-certify a variety of cylinder and gas types, including CO2, Propane, Nitrogen/Inert Gas.