For free or a small fee, many retail stores (Walmart, Lowe's, Agway, etc.) that provide propane cylinder refills will trade your obsolete cylinder for a filled cylinder with an OPD. We recommend calling the store before bringing in your propane tanks.
Many gas stations (Kwik Fill, Fleming Pit Stop, etc.) that offer propane refilling may also take your cylinder for trade in. We recommend calling the gas station before bringing your propane tanks in.