Rimmers seem to have the LHS tourer body panel(including sill)pricey though.
https://rimmerbros.com/Item--i-ALA500370
I would imagine the sill section is the same as the saloon.
Having said that,not meaning to blow my own trumpet,having done one of my own sills last year,using a little cardboard engineering to get the template,,i was very pleased with the results.And that was with a cheap gasless MIG,bought for £78,Yes,there was a little weld splatter,but the grinder smoothed that off and a slight amount of filler ,just to smooth and blend in,everything was fine.
It is worth a go,,just practice on a bit of scrap,and have a friend stand by,in case of fire,although I did pull back the carpet and trim in the area above the repair,just to be safe.
Hope this helps,,Ken.