Mint Ikebukuro is my go-to store as far as new release singles comes. They have singles of every release, including the box sets, and they're pretty easy to find in the store. The store itself isn't too far of a walk from Ikebukuro Station, though Ikebukuro is about 45 minutes from my house. I try to stop by about once a month these days. These are my pickups from mid-October.
Next time, I'll have some more pickups from Ikebukuro - but from the other shop across the hall. (What? You'll see when I explain.)