Why the plug? Did you lose it, is it stripped, cross threaded?
As far as the "gasket" goes...do you mean the whole pan gasket or just the washer?
The washer can be had at any hardware store...pick an appropriate size...you can use copper, nylon, fiber. It may be a specific part number but any sealing washer will work fine.
You can also try to match the whole bit up with different sizes of oil pan bolts/"gaskets".