How to identify/find replacement bolt for STI
You should be able to source a replacement many online industrial parts suppliers. You may not get an exact replacement, but you should be able to get a bolt that works. 

Presumably there is a identical bolt on the other shifter. You can use that to find the diameter and length you need. It will be a metric size. I believe that small metric bolts only come in 'standard' pitch. You'll want to get a bolt in stainless steel for corrosion resistance.

In the US McMaster Carr is a well known parts supplier. https://www.mcmaster.com/stainless-steel-bolts