LFA5500 dates to 1978/79 vintage ... washers of this belt-drive design went out of production in 1986 so majority of parts have gone out of availability except for what's left to be found stashed on shelves or gathered by collectors, and the occasional eBay offering.
A gasket can be cut from cork material available at automotive parts suppliers.
Go to the Sears website and look up the part numbers. Google the part numbers and you should have an easy time finding them. This website is an excellent resource as well.
I swear I think I saw the gasket for the belt drive trans on eBay for just a couple bucks.