One way is to hire a small Transit sized pick up truck with a demountable body...arrive on scene, drop the body off on a piece of level ground, pop your tarpaulin in position and fill it up...if you ask nicely, a local farmer may have a water bowser he'd be willing to use to fill up your makeshift pool for the price of a bottle of scotch...
Or, if you ask very nicely and you're at a show for charity, the Fire Brigade may come along and fill/empty your pool....again, a bottle of something suitable may help.