Air muscles aren't a very common item - not the sort of thing you'll
find in a shop.
The Shadow air muscle company are in london, but they would prefer to
build an entire hand and are mostly for research.
Festo make the most common air muscles, but they're in northampton
(and germany) and would prefer to send a salesman round.
Your best bet is possibly Mindsets (formerly MUTR) who seem to have a
couple of air muscles, but they look like home-made ones, not in the
same class as Festos. I have seen Mindsets at the occasional
exhibition so they might be willing to let you visit. They're in
Herts, and related to Middlesex University.