A lot of craft brew places sell 64 ounce bottles of their brew to go. They use the tap to fill a glass bottle then screw on a cap.
This works great if you finish it in a day or two, but for me, unless someone is visiting, I might have an 8 oz glass with dinner. So in a few days, about 1/2 way through the growler it's going flat.
I recently bought a stainless version (the picture is big but it's still just 64 ounces) that has a CO2 cartridge that keeps the beer pressurized:
It was in the refrigerator while I was travelling this past week. Last night with dinner, the head was good and the carbonation was like new: