Many of the mail-order seedhouses offer cover crop seed in smaller amounts. The specialist in unusual cover crop seed is Albert Lea Seedhouse in Minnesota. The shipping cost, about $70 per 50-lb bag, makes it most appropriate for smaller scale trials.
Local farmers and dealers may have farm-produced medium red clover, small grain and buckwheat that is suitable. Make sure it has no weed seed and germinates well. Not sure? Contact the New York State Seed Testing Lab. 315-787-2242
In New York, the suppliers below have cover crop seed for bulk purchase. These are often the primary distributor or actual producer, which keeps the price down. The delivered cost may be lower through your local farm seed dealer.