If you look in Farmers Guardian you can get 1,000 litre tanks with taps on but you'd need a little trailer for towing it. The tanks are about £40-£50 but the trailer would be extra. The tanks are white plastic inside a metal frame.
Or Google tanks that can be pulled by a quad bike to get some names and then look in Farmers Guardian.
Templars your talking about IBC tanks, my sister has them & they are really handy, but yes like you say you will need a small trailer to tow it. They usually have quite a few on ebay for sale, i also know someone who sells them for £20 each, but they are just outside of Caerphilly, soa bit far to travel from England lol!