Let's hear it from our new crowd, do you have a reliable corner shop supplier or perhaps you order online? Maybe Sainsbury's has all you need, I'd still love to know!
Mostly I buy from Yuuki-Cha.com for organic Camellia sinensis Tea in a Japanese style, and Japanese teaware. And for other herbs, I search the world over for good quality, and reasonable prices.
I'm mostly living in the green tea world, so Ippodo and Den's Tea are my go to. They both have some darker tea offerings, like fermented Puerh or Oolong. Mostly looseleaf. I'm not very in the know, but they both seem to be good suppliers.