Steve
The mic is the same as for my M603: (mine's a HM-126b but presumably merely an earlier model) and will surely be available direct from ICOM. If not, it's available in the UK for around £56.00 from:
Icom IC-M505 Accessories, ML&S Martin Lynch and Sons Ltd
I have twice had a similar problem with mine from water ingress but with no internal physical damage (just shorting causing channel changes). Each time I opened it up (four screws on the back), left it in the airing cupboard overnight to dry out and reassembled with it working as good as new.

I need mine today but if you want to borrow it during the week until you get yours mended/a replacement, give me a call.