Rexhall RexAire parts

Rexall RexAire 2010 with gas Workhorse engine:  Error message Hydraulic Brake Warning, Check ABS.  Has been diagnosed as accumulator balls need to be replaced, but they are not sold separately.  The entire Valve Assembly needs to be replaced.  We need help to locate Valve Assembly.  Our Mechanic and people at WorkHorse have been unable to find parts.  Can you direct us to locate for new or rebuilt?