I believe the standard bolt size for the ones that mount the caliper are - M6 x 18mm.
Dead easy to find, especially in stainless or Ti.
I replaced all my bolts with stainless ones, never had a problem with them. Lot of companies stick stainless bolts in their high-end products eg. stems, pannier fastening kits etc.
Wouldn't bother with Ti, I've stripped a few in my time and never bothered going back. Did work out about 40% weight loss with Ti, but just don't trust them anymore.