Where can I buy specialist car parts?
With the cult status of many Japanese performance cars, is there somewhere in Japan where tourists can buy spare parts or performance items for cars, like specialised steering wheels, gear knobs, suspension units, etc? (Preferably somewhere with English speaking staff?)
I'm not sure you're going to find an enthusiasts store or website in Japan that's not in Japanese. But if you don't mind a bit of body language, most auto parts stores staff are pretty friendly.
Probably you should start with Autobacs. This page is for their tuning and performance parts:
Besides them there are plenty of others, but Autobacs is the biggest and best stocked. Here is a list of their stores around the country.