I haven't had any issues finding the drill bits - they're Hilti, and available in many places. They're not cheap, but last for ages and drill a very round hole. However, I also have concerns about the dirt ingress issue - not so much inside the hole itself as that can be cleaned out with a blower and brush, but it's the dirt and grit picked from the rock surface, gloves, etc. that will be the issue It's very rare to have a perfectly flat, clean and dry surface to bolt into underground, and without taking clean water and a brush/rags with you, there's not really any way of fixing it once you've got started.
Also I didn't realise they were ?40 each. The screwbolts I use are around 45p each, and are thrown away after removal, but I've got through well over 100 on our current project so far. I'm sure the cost can easily be justified for work, as long as they're looked after, but given the way many RA techs treat their kit, I'm not sure how many would survive for long in that context.