I've replaced mine with stainless steel allen/socket heads, as above, they are M6x1. Next size up is M8x1.25, these are the diameter and thread pitch in mm.
Thats all there is to it! some metric bolts have a Fine thread, but I don't think there are any on our bikes. I just call round to the fastener shop that happens to be 2 minutes walk from me, and buy stainless socket heads, usually 10 or 20 of the most used lengths, cost me less than a penny each.
Thats the beauty of Metric: very very simple. One day the USA will catch up with the rest of the world and go metric