It depends on if you have original valves or if they're been changed out.
The originals I've fixed on my tractors basically used an O ring for the packing. IIRC you will need a small diameter but fat O ring and may have to check several places. I think I got mine at an auto parts store.
If they are aftermarket (most likely Rego, but other brands out there) You may be able to buy a packing and seat assembly that will fit your valve. Last time I worked on one like that it cost almost as much as a whole new valve, but sometimes pulling the valve body out of the tank can turn into an unwanted adventure. Check with your local propane dealer.