If you want something you can keep on your person you can look into the new SIGG classic bottles, they have a cup that is shaped like a pocket bottle (where they keep alcoholic drinks in) and I guess it is about 0,4 liter in size an can be carried in a large trouser or cargo pocket.
http://www.sigg.ch/en/html/images_sports/ov_blackretro.jpgRick