Western Gun Parts, in Alberta has the RH Browning Challenger grips, but they are more of an ambidextrous grip.
The Browning Medalist had a RH or LH thumb ramp, but were not adjustable. The Browning International Medalist had RH or LH target grips with an adjustable palm shelf, so I am assuming that is the grip set you are looking for. I'd still try WGP for them.
Phone: (780)-489-5711
Aftermarket grips may be the best bet if the Int. Medalist grips are not available.