Hobart's website has an online catalogue, if you want to deal directly with the source. You'll need to register with them in order to browse the catalogue, and download service & repair manuals. Otherwise, you can try calling their Customer Service number at 888-4HOBART (888-446-2278).
Alternatively, there are a number of suppliers that sell the equipment and parts.
Valiant Equipment in Florida is one of them.
If you bought this piece of equipment new, you might try contacting the vendor from whom you purchased it. (They will probably have stuck a decal on the unit with their name & phone number, hoping to get more of your business.)