As far as I'm aware, TGN is the only independent, reliable, affordable supplier of a huge selection of shaving brush knots, in single or low volume quantities. Apart from that, most brands will supply individual knots. I'm pretty sure Shavemac (DE) and Vulfix (UK - actually Isle of Man) do; others will undoubtedly also do. Other than that, you'll have to order in bulk (or request samples) from China.
I'm not aware of any 'independent' (i.e. not a brush manufacturer) UK supplier of knots; in the US, several woodturning suppliers do, but none has the selection, quality or price TGN does.