I'm not a violinist, but I've heard left handed people have and advantage on normal violins and violas because of the left hand is used to finger notes on the fingerboard. Right hand is used for bowing. If you played a "left hand" violin/viola you would have to learn everything opposite and it would be very difficult. I think that's why you can't really find them anywhere. All the left hand string players I know are just fine playing on normal instruments.
Hope that helped
They do make some violins to be played left handed,but you can set up a regular violin to play it left handed too. You hardly ever see them . I think it is simple to know why if you think about it. What would that look like in an Orchestra?
During Vivaldi's time, left handed people in Italy were considered bad luck to their families. Since then, the use of left handed string instruments are not acceptable in today's orchestra society. Being left handed and learning to play the violin is possible and can be done, but it is actually slightly harder than being right handed, since your right hand is your bow hand. Hope I could help! Best of Music!