Depends on how much you want to spend. You can try those water bottles used by long distance runners (those that they strapped to their running belt) or you can try this product call Platypus soft bottle (I saw them in Isetan), they fold flat!
Might be worth a look I guess, since sometimes I only need that little bit of water so I'll fill it up halfway or something. How much didja get it for? At isetan? Any leaking etc? Cause I'm gonna be putting it next to a camera eh.. I know those small nalgenes won't leak.