Many of the brakes use the same size threads in the adjuster M5/M6 for road bikes. If you have the old broken part & adjuster nut, take it to the bike shop and see if they can match it. Shimano may use the same size and you could reuse the existing nut which has a wavy shape on the edge that sits on the top of the caliper arm & provides the detent as you turn it. The shop may even have some discarded brakes in their parts bin with the same size. Usually they're alloy so should be easy to file down.