Hip flasks were traditionally made of pewter, silver, or even glass, though most modern flasks are made from stainless steel. Some modern flasks are made of plastic so as to avoid detection by metal detectors, the plastic flasks also have the advantage of a lower price
Hip flasks can vary in shape, although they are usually contoured to match the curve of the wearer's hip or thigh for comfort and discretion in a design also known as a kidney flask.
Some flasks have "captive tops" which is a small arm that attaches the top to the flask in order to stop it from getting lost when it is taken off.
A hip flask is most commonly purchased empty and then filled by the owner. However, the term "flask" also applies to smallest bottle sizes of alcohol in commercial markets. Some flasks come with small cups to make sharing easier, although generally liquid is drunk directly from the flask.
you can find a big variety of flasks here
we also have a nice collection of flask gift sets, that include funnels and shot glasses.