well, you can get some of the strongest if they carry them, as they are sold in bulk.
Samuel Gawith has a few that are pretty strong, namely: 1792; Bracken Flake; Brown Rope #4.
Gawith Hoggarth & Co has some of the strongest: Sliced Brown Twist (or Brown Twist Sliced); Dark Flake unscented.
there's a site that has their own "strength meter"... gives good descriptions as well.
http://www.synjeco.com/pipesandtobac...tobaccoall.htm