So, are you willing to buy from another individual?
If so, search by state here for NPIP hatcheries and breeders.
http://www.poultryimprovement.org/statesContent.cfm
They use stock codes. The codes for the Plymouth rocks are as follows:
Good luck.
Can you provide more info? Location, what Dick Horstman sells, etc.
What are you looking for with regards to poultry: eggs, chicks, started pullets, etc?