So here are two options that i'm fond of, though admittedly i use them for at home storage most of the time since cards on my avchd cameras last so darn long. But if you're shooting RAW or otherwise high bitrate formats which eat up cards quick, then you'll likely need a bunch on hand. Enter the two cases, the larger one being the beefier, more protective one that can hold twice the cards. Both of them hold CF and SD cards, twice as many SD than CF in both cases. While the larger case can take more abuse, the smaller one seems perfectly suited to handle day to day environments.
That about does it, they're both cheap, so might as well get both! both can be stuck in a pocket, hung by a chain, etc... though i'd advise against the latter.