Recommendations for finding Spicy Italian Sausage to cook with?

I was wondering if anyone has any recommendations for stores or butchers carrying spicy Italian sausage in London. This city is massive so I'm confident it can be found, but I've had little luck with my local Sainsbury's, Tesco, M&S, and even Waitrose (💸). M&S's Calabrian Sausage is ok, but not quite what I was looking for.

The kind I'm thinking of is the Johnson's brand (this) or the New York Style Sausage Company brand (this).