If you're going to Akiba anyway, try the big AsoBit store on Chuo-dori and the Gamers store (a few steps to the south/right of the Akihabara JR Electric Town exit - left side). AsoBit has most of the mainstream stuff.

Also consider wandering the small streets to the south behind the main Chuo-dori drag. Lots of smaller shops carrying an eclectic collection of dvds, figurines etc. Not much in the way of fancy packaging since most of the stuff is truly "collectible" (out-of-print or discontinued lines/characters). Some things are discounted and others are laughably expensive.

I'd advise not bringing the kiddies off Chuo-dori because you'll likely encounter unexpected surprises related to the local taste for underage fantasies.
<puking emoticon>