Decades ago, in 1975 I went to the an Aircraft Surplus place and found plenty of allen key fasteners for Campagnolo components.
for the cable bands and front mechanism band clamp I went to 10-32 in Titanium no less with matching Nylok nuts. Not too hard to find 5 mm metric today in ti.
for the cable anchor bolts all mechanisms, 5mm socket head fasteners in Stainless steel. ( check the 'bay for them in titanium today, also work for waterbottle braze on bosses (most) , toe clip fastening bolts.
Look for AN washers, not SAE, the AN washers have better tolerance.
Some noticed, many did not.
Harder to find Stainless steel screws for top tube cable clips.