Have the dealer order it from the states . I've odered lots of dealer sourced parts that had to come out of the states , it usually takes a week or less.
These snap rings are known to pop out of their groove , also , an overhaul kit ( not a dealer item ) should contain that ring , be advised these snap rings are selectable thickness , as in , the correct ring will set the end play of fifth gear , the dealer partsman will ask you which snap ring you need , there are a variety of thicknesses available