It is very easy to change the gasket at the final drive’s rear plate without having to remove the gearbox or anything else.
A couple of years ago i have bought sump gasket from Speedparts,
but now i seems like you have to buy a complete kit to get the gaskets. Contact them and ask, maybe they still do sell the gaskets you need separately.
To drain the ATF oil from the gearbox you don’t need to remove the sump, there is a small drainplug at the front of the gearbox. On my cars the plug opens with an ordinary screwdriver, and it doesn’t really look like a drainplug.