Where can i buy liquid mercury from?

Answer 1

Firstly a barometer does not use mercury to measure atmospheric pressure.

Secondly you need to have a license to handle mercury since it is a very dangerous material.

Some barometers do use Mercury, but not to directly measure the atmospheric pressure. It was used because air did not mix with the mercury because of the density.

So the air would "push down" on the mercury, pushing it up another tube that had a measuring system on it, usually metric.

This is where we get the pressure unit Torr, which is 760mmHg, which is 1 ATM (atmosphere), the pressure at sea level.

Answer 2

You can try in a chemicals store or by order from a chemicals producer. Depending on the country mercury is not accessible for all persons.