I don't know about you, but I've never been able to keep a leftover slice around long enough to cart it anywhere in a little resealable pizza case. But, if you have such willpower and you just happen to live in Seoul, South Korea, you might want to pick up such an item. Slice found this one on the blog Superlocal via Poketo. It was purchased at a thousand-won shop there, the equivalent to a dollar store in the States.
Update: As homeslice Prairie points out in the comments here, yes, they do have something similar in the States. And I blogged about the Tupperware pizza container way back in 2003. Although Tupperware no longer offers this item, you can almost always find them on eBay.