Call a local propane/gas business and find out which of the local shops does tank maintenance. Take it by there.
As an aside if you go to an Ace hardware store they will have a, what I call, a wall of parts. Row after row of cardboard drawers. Some Aces have bigger walls than other. And in some of those cardboard drawers you will find o-rings and gaskets. I don't recommend doing that for the propane tank since it's a safety issue. But next time you are in an Ace just take a look at the wall to see the wide variety of stuff they have. Sooner or later you will want a knob or spacer or who knows what else.
