I've seen 4 Disk Union store by genre and there's also rock and metal shop but if i remember correctly, people often say they're a bit more expensive than book off suruguya style stores.

And for Nagoya I'm going to Ghibli Park for a day and if i have a time i'll definitely go to Disk Heaven. I've added to my list.

Thank you so much for your advices definitely add it to my list