Shibuya Parco is well worth your visit (it's a ten-minute walk from where I commute to daily) as they have an anime-themed sixth floor, including a NoitaminA shop. But I don't believe there's a Shirokuma Café store there, or anywhere else I've seen. I've seen merchandise on sale at many places like Animate and Cospa (the Parco might have a store that sells some) and a café was doing a Shirokuma-themed menu, but I don't think there's a dedicated shop.
"No matter how wonderful a guy may be, his tail has to look amazing - especially how he swings it."
