Hey: I started my son on a 150 Burton Alp last year, he was 9 and about 4'7" tall. It was over his head, but he picked it up quite quickly. There is one on ebay right now. The race plates go down to a mondo 21 boot.
http://cgi.ebay.com/Burton-Alp-50-Alpine-Carving-Snowboard_W0QQitemZ220171043935QQihZ012QQcategoryZ36297QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
I don't know if you have local ski swaps, they work too. My 8 year old wants in on the carving scene, I found a Nidecker 140cm board for $35.00, never used. Anything below 150 is almost impossible to find, I looked for two years, the Nidecker was a fluke. Good luck, and keep passing on the rush!!!!