The brake shoes and the brake pads will be unique to the model of brake.
For instance the Hope hydraulic brakes have about a dozen different shapes and sizes.
Avid are the same:
Avid BB7
Avid BB5
What you need to do is identify the make and model. Once that is done then most bike shops will have pads for the common makes and you can order most others on line.
Even if this is not possible as the pads have been withdrawn, then the good news is that the fitting on to a bike is more standard and there is no issue in replacing the whole system.
New hydraulic brakes tend to come in standard lengths, and cutting involves bleeding which is easy enough but messy and difficult the first time. Cable brakes like the Avid BB7 are easier to set for unusual length
Edited....
Of course the early Brox were very individual and used a lot of custom parts. There are a couple of owners on
YACF who may be able to advise if you get stuck