Fedco Seeds is the best source at this time. They can be found online. They are a major seed company in Maine, USA.
The original source for Duborskian Rice is Sourcepoint Seeds in Hotchkiss, Colorado. A much smaller company but might be worth a try.
Either way you are probably running short on time for this growning season, as they need to be started in flats for 6 weeks, and then 3-4 months of warm, dry weather to mature.
Good luck in your pursuits!