Dunno if this helps, Paul, but there are two allen bolts on my brakes:
The larger one in the middle is easy to replace, pretty much a standard thread. This holds the whole arrangement onto the brake caliper.
The smaller one on the right prevents the brake pad from sliding off the holder. I always round this one - and I don't know how you get replacements.
If in doubt, try your LBS - they should have drawers full of spare bolts & bits.