This happened to mine as well, but the green button perished only in the centre and this bit ejected itself into the nether regions, never to be seen again. I stood there holding the green bit in my hands wondering what I'd use to secure it and feeling well-chuffed I'd not lost it. I was able to use some 3M brand yellow weatherstrip cement ('Gorilla Snot') such as what is used to secure windscreen and other mouldings and such on car bodies. A black version is available as well. I simply snipped a piece of thin hard plastic into a circle and glued this with the 3M cement to the outside of the remaining green button with the hole in it, which allowed a bit of cement to pass through and in turn is now stuck to the red bit below. It has held for nearly a year now. I then sourced a clean, secondhand switch assembly from Motorworks UK for something like, I dunno, maybe AU$45.00 and this is in the garage for the inevitable replacement down the track some time. On tour it's in my pannier...
For those West coast Canadians who can't find what they need in the Great White North, you can always 'take off' and try that bloke in Seattle, WA who sells only used K bike parts. Try http://www.kbikeparts.com/