You may want to try Kitazawa Seed Co in Oakland. They specialize in Asian seeds as well as some rare Asian vegetables you cannot find anywhere else. You can visit them at www.kitazawaseed.com

Their 2007 paper catalog lists Rakkyo bulbs on page 32 and they are shipped in September. I have ordered seeds from them and they are one of the best companies I have dealt with in both customer service and results. Definitely worth checking out.