I have six 20 pound tanks and a 40 pounder that my Lions club uses for monthly barbecues. I take them to my local farm supply store for refills - cheaper than the propane company that supplies the store. They check the dates on every tank every time.
Last year, the 40 had to be recertified. The farm store directed me to my choice of two different propane suppliers. I already knew that one (the one closer to home. of course) charged $40-$50 and you had to leave the tank until someone decided they would look at it. I went to the other supplier and got the tank certified and filled at about the same price as just the fill at the farm store. I'll be going back there when the 20 pounders need to be done.