jenny....I presume you mean a static van and not a mobile one ?
Find a site that you like in the area ( I have never been there myself ) There will almost certainly be some vans up for sale, especially at this time of year. Some will be owned by individuals and some by the site owners themselves.
All sites will charge you some kind of annual site fee, on top of the sale price of the van, and these can vary quite a lot. The more facilities that the site has, its likely that the site fee will be more expensive.
Most sites have a 10-11 month occupation rule, to stop them being classed as permanent residences. But that isn't really a problem, as the off months are normally January and or February.
Also, most sites have a ruling that says that you must renew the actual van every so often. This is to keep up the general appearance of the site.
An alternative to a static van is a mobile one, like mine, which I keep on the site 12 months a year. The site owners move it to a pitch whenever I am down, and put it back again afterwards to the storage area. This is much cheaper than a static van, although you will pay the standard overnight fee whenever you use it. My storage fee is only £21 a month, which is easily affordable and it costs about £15-£18 a night when I use it.
I don't know whether any of the above is of any use, but feel free to ask more questions if needed !