I have a bunch of spare parts, but purchased them all as complete Swiss parts kits. One option is to contact Colorado Gun Sales. They seem to have spares, albeit at very high prices.
Something to consider - a good friend of mine who is the Caribbean LE distributor/rep for Swiss Arms saw my collection of parts for the 55x series when he was visiting the US. He laughed and told me I was wasting my time/money; that he has never had any part fail on any of the 55x guns that have come through his hands. He doesn't exactly baby his firearms and his clients are mainly Caribbean drug interdiction teams.
That being said, I like the idea of having a spare bolt, carrier, cocking handle, piston with return spring, gas valve, and select few small parts such as various pins and springs.