I think that Fons has probably the most rare:
Thamnophis eques cuitzeoensis
Thamnophis melanogaster canescens
Thamnophis fulvus (if I recall correctly, also from mexico)
I think you'll have a hard time finding them in the UK. I believe this year was the first year that was bred with the first 2 by Steven Bol and he did not have a hard time selling them 
. The fulvus come from Germany.
Next year, I'll probably have Thamnophis eques cuitzeoensis also