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I'm thinking that as long as the plug WILL tighten up/threads aren't stripped you can use the same gasket that is recommended for the engine oil pan drain plug:
I have sitting here on my desk an OIL TITE brand #65269 metal/rubber gasket that I use on MY oil pan drain plug-I change IT every other oil change-they are cheap enough-just remember to remove any other washer or gasket from your trans pan drain plug......these are also made in 1/2" size...the one above is 9/16"
I've never compaired the two plugs, I use the M14 metal/rubber gasket on my ENGINE OIL PAN drain plug--the drain plug on the trans may be a 1/2" and in that case there are 1/2" metal/rubber gaskets available as well.
The washer/gaskets with the metal/rubber are fantastic! not that expensive and most parts stores have them. Just be sure to remove any other washer/gasket that is on your drain plug.