There seem to be dozens of groovy little gift shops that sell various gift mate cups. I see em everywhere in the "tourist crescent" from San Telmo to Belgrano, but even in neighborhoods like Villa Crespo or Caballito there are high end housewares shops.
I have seen hair on hide leather wrapped ones, silver inlaid ones, carved gourd models, plexiglas, ceramic, and solid silver versions, and lots more.
Usually stores that sell fancy candlesticks, stemware, and cutlery have them.Also many stores that sell folk or country crafts, such as belts, bombillas, and carpincho loafers. Sometimes tobacco stores that sell high end pipes and pocket knives will have em, too.
If you just walk around either in Buenos Aires Design, the fancy furniture and accessory shopping mall under the Recoleta Cemetary, or in Palermo Soho, you can find lots of cool ones.