I thought you could adapt something/just use the boot from one of those buttons. The switch box is available on PartsTree for $137. Otherwise it's impossible to buy just the button alone. Check local ads - Maybe someone will be giving away a dead starter and you can snag the harness and box.What do you mean by it is not serviceable? I had the housing box off, opened and old boot removed in about 3 minutes. I assume you are just repeating what a MTD or Tecumseh service manual may say? They may not sell a replacement part. That is what I would expect at this point in time.
Craftsman2: Looking at a place like McMaster is a great suggestion. However it is not a threaded mount boot. The boot base is a rubber ring that is clamped in place between the switch and housing.