Cockpit, Yellow Hat, and Autobacs are automotive chain stores. Sanwado and Sunday also carry some minor accessories... but the large Sanwado "Dream" stores have full featured automotive sections with just about everything.

if you can make it to Honshu, Aomoriken, Hachinohe City by taking a ferry from Hokkaido, you can hit up plenty of tuner shops there. Joule is one of the more popular in Hachinohe City, and Auto House Mirai in Aomori City is guaranteed to have the parts you're looking for in-stock.

from Hokkaido, the ferry will take you to Mutsu. you'll have to take a train for about 2-3 hours to get to Aomori City, which is east south east of Mutsu.

consolidated list:

retail chain stores (like pepboys): Cockpit, Autobacs, Yellow Hat
retail stores with autoparts: Sanwado, Sunday
Hachinohe City: Joule
Aomori City: Auto House Mirai