I just topped mine up again. It was at "400 miles before no engine start" warning and took 10l container. I have been buying double packs of Greenchem ad-blue from Amazon at £17 for 20l but won't need that much again before the lease is up, so have looked around.
You will get ad-blue on almost any forecourt, but beware of any that has been stored for long periods in the sun. Also available in Home Bargains (5l containers), any motor factors, online at EuroCarparts and Screwfix - to name just a few. If you are passing any of those places, get some. However, I doubt you will get a full 10l pack into the tank with 1300 ad-blue range showing.