I get it in 40 gal drums for my truck, but thats not practical for a van! DO NOT buy it at a garage...they will charge the earth. Cheapest places I have heard to get 10l cans is screwfix or toolstation...have a google! Presumably you have got a gauge for the AdBlue tank. Don't let it run out...it can be an absolute pain to sort the system out afterwards. AdBlue does not actually go in the engine, but is used to clean up the exhaust gases. My truck uses about 15l a week, but I'm sure your van will use far less than that. You get a blue nozzle with the can to help top up the tank. If you spill any, it can leave a white residue, but this easily washes off.