Hi Aldus,
In addition to the 'Banjo' voice in the 'String' folder of the Panel voices mentioned by Drake, you will also find additional voices 'Banjo' and 'MutedBanjo' in the 'Ethnic' folder of 'GM&XG' voice category, plus 'Banjo' in the 'Ethnic' folder of 'GM2' category.
Regards
Ian