I purchased birds in the early days from Sandhill preservation but didn't use any because after growing them out I realized the quality was not to SOP, including white legs! Currently, I have birds from Rich Lehnert and JT Cashman in my flock and hope to introduce another line this year.
Yes my adults are F1 and there is still work to be done in creating a perfect SOP. My mentors and I spend a lot of time trying to track down people that are raising this variety. Most connections have led us to flocks that are not being bred to the standard and most with white legs. At the beginning of my search I contacted breeders like Curtis Oakes and Paul Gilroy trying to secure some birds, but alas their flocks fell prey to predators.
It has been quite a journey even getting these birds, and a continuing quest to find better stock. If you are aware of other sources, I would welcome the information. After all, my end game is to bring these birds back to the glory they should be celebrating.