Not sure what the gasket is. I would imagine it could be a brass crush washer. You would probably be able to go to your local hardware store to find a substitute, but the issue is you could only do it with the gasket in your hand and your car empty of oil.
Worst case is you reuse the gasket. I've reused plenty of gaskets on oil drain plugs in my time, and have never had an issue.
On another note I just changed the vacuum pump seal, super easy. Took me maybe 10-15 minutes. The old seal didn’t seem too bad, but I did notice it was much harder than the new one. Fingers crossed that I fixed one leak.
It is a rubber seal built into the oil drain plug, not replaceable IIRC. I re-use it, no issues so far.
I'd recommend some sort of plate or baffle to direct the oil stream coming out of the pan, it makes an incredible mess otherwise. Last time, I tried a 1-inch 90° PVC pipe elbow, put in place as soon as I pulled the plug. Reduced the mess 90%.
I will third the re using the drain plug and not having any issues and second the notion of having some sort of system for oil drain management as I made a huge mess the first time.
Our last LR4 leaked slightly at the passenger rear valve cover gasket. My notion is that it gets cooked by heat from the exhaust piping right below it that is slightly different on either side of the engine.
Thanks for this thread, very informative!