Chris Shaw www.shawstainless.co.uksells them in Stainless Steel, along with lots of other lovely Stainless fasteners and bits for Airheads. Mine arrived this week
(Edited to record that I didn't read the question properly, and wrongly posted about the screws that hold the 'Vaccuum chamber' on to the top of the carb. D'oh!)
Not certain about this but I think that the thread is a lot finer than the standard M3.5, I only mention that in case you bugger the thread up while trying to make it fit. Or in other words, be gentle with it.