A "Dewar flask" is a double-wall flask where the inner space is evacuated - it is also called a "vacuum flask" or "Thermos" (usually the latter if sold for keeping food hot or cold). This was invented by Sir James Dewar back in the 1800s.
In the lab, it is often used to store cryogens, like liquid nitrogen. The vacuum provides thermal insulation from the environment. Lab Dewars are typically silvered - this increases the insulating power.
Typically you would get a Dewar from a glass company - Ace Glass and Kontes are two big outfits, or big lab supply companies like VWR or Fisher also carry them. If the one you need isn't stock, they will make it custom.