-  I have been advised to practice against a root and fofth "drone" as a method to improve intonation.

   The following possibilities have come from Internet search and personal recommendations:

 
http://www.google.com/search?as_q=electronic+box+&num=100&hl=en&ie=UTF-8&oe=UTF-8&btnG=Google+Search&as_epq=&as_oq=sruti+shruti+&as_eq=&lr=&as_ft=i&as_filetype=&as_qdr=all&as_occt=any&as_dt=i&as_sitesearch=&safe=images
Google

U.S.A.

      http://www.indousplaza.com/Dhruva CD Deluxe $125 .  CD Excel II $130. SPL $ 135
                Manhattan  108 East 28 Street (Bet. Lex. & Park Ave) New York, NY 10016
                 Tel: (212) 684-4447
       http://www.musicoutfitters.com/ethnic/elsh.htm    . Ely, Minnesota  $150 Dhrva CD Excel
       Freeport Music -  41 Shore Dr HUNTINGTON, NY 11743 631-549-4108
       Buckingham Music - Austin Texas

U.K. ??
        Offer.com - Radel Shruti Box

Germany
  Tarang Indian Instruments . Raagini Sur Master professiona

India
       Radel India   -  Bangalore  -  Has Sample tones .
       DMS-BINA - Delhi - 110 006. India