Viz manga books are notoriously hard to find; Forbidden Planet don't seem to be able to get hold of them but Travelling Man sometimes have a bit more luck (I'll be in Manchester at the weekend and will no doubt drop in on TM).
Honestly, it's infintely easier to go onto Amazon or some other online stockist - high street shops, through no fault of their own, have a really hard time getting hold of Viz stuff.