The gauges themselves appear to be in plentiful supply, being common to many types, and there does appear to be a fair few around.
Interestingly, having recently remade the Hydraulic Panel and now completing the Hyd system rebuild on 558, I recently called up what I believed to be the panel for the Brake Accumulators, with the gauges being listed seperately in the parts schedule, and was surprised to discover that it came complete with the gauges and associated piping already attached as a complete assembly. I was also surprised to find we had a further three in stock, and they had sat unopened in their respective boxes since 1963!
Flipflopman