As far as I am aware passata is obtained by crushing tomatoes and then sieving the result without heating. That sounds an awful lot like juice to me. What is the difference (if any) between passata and tomato juice? Under what circumstances could they be substituted for each other?
Pubs often sell small bottles of tomato juice so on camping trips etc I suspect it may be easier to get tomato juice than passata, although it hasn't been necessary yet.