Take it to a shop and have them tap it and put a plug on the end. By the time you pay for all the tools to tap the pan you could have had someone do it.
You can get one from MSC for about $6, plus shipping. You'd be better off getting one from a local Industrial Supplier. Just look in the Yellow Pages under Industrial Supplier.
FYI- so you don't get a smart-assed counter guy pissed off, make sure you refer to the tap correctly (sometime these guys can be cranky...). You need to not only specify 3/8" NPT, but also need to include the TPI (threads per inch) designation. A common 3/8" NPT thread has 18 TPI.