If you want minimal design just go to Uniqlo.

Tokyo has a bunch of stores that cater to different fandoms, so you really have to just walk around to find them. Places like the shops in Akihabara, Sunshine City, Skytree Mall, and Don Quijote are where I'd find most of my shirts. You could really shop anywhere though and find some anime apparel.

I don't think you will get an answer here. Maybe try on misskey or mastodon with japanese hashtags.

I would look for second hand clothing shop if you want something on a budget.