Weber Factory Store in Palatine IL does sell the 5# tanks. They are about $55.
Local Ace Hardware won't refill them without OPD. Also, they are certified good for 10 years from manufacture, after which they must be recertified. Ace won't recert, but probably if you go to a more industrial propane provider, they will be able to pressure test/check for leaks, and recertify the existing tank. Most rental cos. (the We-Rent-Everything places) can refill propane, ask them if they can recert.
My buddy bought a '99 Performer on CL. Ace wouldn't refill without OPD. We said How can you tell? They said by the valve handle: OPD is the triangularish handle, whereas the '99 tank had a 10 or 12 star valve handle. But, the tank had a sticker stating it did indeed have a OPD, so we were able to get it refilled. Bigger but, as a '99, we won't be able to get it refilled next year unless it is recertified. Or buy a new tank. So, that's how I know they do have them available for purchase.