You can get loads of stainless nuts, bolts, machine screws, washers etc. on Ebay. You can buy one or two at a time or some sellers will do selections of assorted sizes.
Anywhere near Exeter? Theres a shop in the industrial area, near the recycling centre, Army Surplus sought of thing with loads of odds & sods along with a decent hardware shop next door that does just about every stainless screw possible.
probolt for Ti and aluminium alloy. like stainless, both are less strong than high tensile steel so giv e some thought as to what bolts you are replacing.