Water Tank For catering trailer

by Jolie
(Essex)

I bought a catering trailer super cheap off of a friends friend. We are a little concerned about where we can find a water tank for it. It has the pipes and pump under the counter and a hot water urn.

The trailer runs on gas, but also has a drinks fridge, fridge freezer a bain marie and a small hob.
We want to put a coffee machine in there, but are stumped as to where to buy a water tank and where the best place to store it is.

Any help will be a great help.
Thanks
Jolie