Call some of the local propane dealers in your area & ask if the recertify tanks. Ask how much. Mom & pops usually work better as they tend to have someone who can recertify. I got my 5 lb tank recertified for free with a fill-up at a local place. If memory serves, it was a bit more than $5 (including tax) and the tank was filled up too.
It had the old style valve handle, not the newer triangular style used now that indicates it has overflow protection, but there was a sticker on it that stated it had overflow protection. The bigger places often won't even fill it unless it has the triangular valve handle. He mentioned that others might not refill it because of that & basically suggested I become a repeat customer. I really don't use propane too much, but will use this place as it's family run & he will fill a 20 lb tank all the way to 20 lb for around 20 & those exchange tanks are often only filled to 15 lbs & often cost 20 & up to exchange.
For anyone near O'hare in Chicago, if you ever drive up Milwaukee Ave through Niles, bring your propane tank & stop at Tebons. Propane seems to be all they do & they have been around forever.