Nest time one of them needs a refill.. Google Propane {zip code} and look for a "Store" with pictures of Propane Delivery Trucks on their web page.
If there are more than one in your area call around.
They generally do re-cert at these types of places.
Have the tank as empty as possible when you show up They will, for a small fee, inspect and re-cert the tank for you (assuming it meets requirements).
I usually get propane at 2 or 3 places,, One of them I've seen do it,, One of them CAN do it but I've only been there once.. The other one can not do it (Hardware store).. The park I'm in now.. Believe it or not, can do it. but I do not buy propane in RV parks as a general rule, (Higher prices).