LondonClassic
Retro Guru
The stiffness you mention could just be dirt so as a first go maybe some 3 in 1 or similar lube down the cable hole in the lever - that can transform shifting. Failing that, I had my ergos brilliantly servced by year1cycles a few years ago - who while they were their fitted record internals into my 10sp Centaur R hand shifter. They can do all sorts of stuff, like add or remove speeds as well.