There are plenty of places to buy Castella, but if you're looking to buy the good stuff, my wife says this is the best one.
If you go in inside, you'll be treated to some nice tea and castella.
What to buy: Castella is a sponge cake popular among Japanese and known as a good souvenir for anyone who has visited their. Some of my students specifically requested that I brought them some. It's ingredients are basically sugar, flour, and eggs but it comes in a variety of flavors. I've tried the plain, chocolate, strawberry and green tea ones.
Originally, the cake baking style brought to Japan by a Portuguese missionary in 1556. The Portuguese Pão de Castela (bread from Castile) came to be known simply as Castella in Japan. The Japanese have a habit doing things like that (i.e. Word Processor becomes WorPro in Japanese).
Anyway, it's a nice souvenir to bring back home. But, you'll have to plan it right as it won't keep indefinately. Maybe it's good for up to 10 days or so.
This is the Japanese shopping mall. Many shops in here like any other mall. You can find just about anything you would want in here.