I put mine in a stainless steel water bottle, then in an insulated lunch bag. Though sometimes I don't even use the bag (but make sure I use frozen fruit so the smoothie is cold to begin with.
You could also use an insulated stainless steel bottle (like those sold for hot and cold beverages) and maybe put a couple of ice cubes (whole) in to keep it colder longer.
ETA: NRGWoman's post reminded me: yes, make sure it's a wide-mouth thermos or water bottle, and buy a bottle brush to clean it.