most of the regulars sell them like omnipole and that, but why dont you set up one yourself, will probably be cheaper. I am setting up a new one at the moment. If you do decide on this option, i have found you will need:
Indespension/ trailer with its own suspension or builders type trailer with carrying capacity about 150 kg more than the water capacity you want to hold, more is better.
The trailer should be braked, unless you have a big meaty vehicle to tow it with or if its less than 750kg load.
Braked as in, with a proper heavy duty tow latch, that works automatically when you apply the brakes when driving.
You should get one with enough space front and back of the tank you fit, and mount the tank directly over the axle/ axles. A twin axled braked trailer would be better than single axle, but its ok if it has big tires, the twin axles ones will carry the load of the water better, as the weight is carried only by the axles, with less load on the tow bar.
your choice of tank, and metal fixing straps. Custom or garage built steel frame section welded to the chassis to support the tank if the flooring is not sufficient. Solid lidded ones are better, or make your own hinged lid that lifts up like a car boot with the compressed things.
I am making the new one with 250l tank bolted down, space for 10 more barrels to make 500 litres. Electric reel at back with roller guides out the bottom. Hinged sprung lid, small overnight electric/parrafin heater, the trailer allready is braked with 750kg load capacity, suspension and is 8ft x 4. Only thing is you might have trouble with turning round, youl get used to reversing them, but on domestic customers small roads, get one the size to suit your work. Ready made wfp trailers are basically glorified off the shelf brand name trailers with all the stuff fitted for you, i think these are in the thousands.