I know you might want to look into getting some-thing you can progressively load for the farmer's walk. How long it takes to max out on regular DBs or even mid-sized bars you can find is pretty quick. I see a lot of people stagnate on their farmers walk, not because they can't progressively train, but because the gyms they go to or their personal gyms don't have the farmers walk equipment. (myself included).
So look into good farmers walk logs or something. It's a killer on the grip.
Also a less common strong-man tool, (esp these days) is a sledge hammer. Just swinging away might not help (probably won't) but levering the thing by the wrist, (building up to that) builds insane strength in the wrists. http://www.youtube.com/watch?v=sowmL2I37wg&NR=1
It also depend on what kind of strongman you want to be, do you want to raise a 300pound log over your head? bend steal with your bare hands? different strongmen specialize at different feats of strength and this requires different equipment.
Even getting a lot of newspaper, folding it up until it's a thick as a deck of cards and attempting to rip through it with your pinch-grip strength can be good training for some strong-men. So, it depends on what your goals are.