Low level tanks are available from a number of suppliers. One that comes to mind is Stream in Stockton-on-tees.
Some window cleaners prefer them because space in the van is not entirely taken up with them, some like them because a low centre-of-gravity may help vehicle handling.
Whatever tank you get, make sure it is installed securely. Water is heavy, and in a 30mph collision, a 600 litre tank will exert a force of 80.4 tonnes on its mountings. If those mountings are made of plastic, heaven help you. Similarly if it is just bolted to the van floor, it will tear free even in a low-speed collision (floor plates or not).
If it isnt bolted down very tightly, there will be shock forces applied to the anchorage points that will greatly multiply the stresses on them.
It must be bolted right to the vehicle's chassis, using enough bolts that are of sufficient strength to hold it there. Self-install tanks are low-cost, but if you get one, I would recommend getting an chartered mechanical engineer to advise how it can be safely installed.
The Institute of Mechanical Engineers will be able to put you in touch with a local engineering firm that can help you:
http://www.imeche.org.uk-Philip