I've got a real soft spot for these things, every bike I've built has obligatory stainless allen bolts wherever possible. They look the dogs danglies.

The best place to get 'em, as far as ah know is

Aerotek.
Unit 64,
Northwick Park,
Blockley,
Moreton-In_Marsh,
Glocs, GL56 9RF

email for above

They also do braided hoses, clips and all sorts of goodies wot'll empty yer piggy bank coz once ye start ye just cannae stop.

Right, so 'ow dae ye work oot whit size o'bolt ye want, well the old Aerotek catalogue had a page dedicated to this very conundrum. Click here for the image of the page it's only 62k but it's pretty big.