For cube shavers, I would recommend either the Hatsuyuki or Snowie brand. The Hatsuyuki brand may be more common and a bit cheaper in pricing, but in the end they shave similar results.
For supplies, I would check out
www.shaveicesupplies.com. Working for the company that owns this site, that would be my only response.
For natural flavors, your best bet would be to make your own. Buying it outside can be really pricey, and unless you plan to charge quite a bit for a shave ice in your store, it won't be as cost effective.