Propane gas valves can be found at any large home-improvement store, such as Lowe's and The Home Depot. Local heating and plumbing stores, hardware stores and farm supply stores usually carry valves as well. Online specialty stores can be a good source of specific valves.
Propane gas valves are common replacement parts. Amazon.com, eBay.com, building supply stores and propane gas refilling stations all have propane gas valves for sale. Propane valves are usually found in the plumbing and heating section at brick-and-mortar stores. Plumbers often work on propane as well, so any plumbing supply shop typically has a selection of propane valves.