At one stage Cloud Nine Farms in Montana used to sell pasture ley seeds. They called it Synergy West Perennial Pasture Seed Blend. The blend contains the following plants; Orchard grass, Perennial Rye grass, Indian Rice grass, Western and Snake River Wheat grasses, Red and Ladino Clovers, Sainfoin, Small Burnett, Annual Sunflower, Birdsfoot Trefoil, Blue Flax, Chicory, Yellow Prairie Coneflower, Purple Prairie Clover, Purple Coneflower, Western Yarrow.
You can contact them here: Cloud Nine Farm Allison and Seann Rooney (406) 578-2144
If they no longer do this, the only other supplier I know of is one in the UK. The website link is: