Sure do. Try a heavy truck like Frieghtliner or WesternStar. They usually carry Mobil Lubricants and GL5-6 95W Hypoid oil is what Rockwell recommends for their final drives. (Of course they are a Little bigger than yours).
You'll wind up buying a quart squeeze bottle for a couple of ounces but you can always use it for bar lubricant on your chain saw (Which I hope is a Stihl)
I would be very aprehensive about buying the oil at a Triumph dealer in as much as they will want to sell you a partial bottle and you never know what crap the wash boy put in that bottle.