Home page, scroll down to 'market place', used equipment is the 1st section.
As for which van, that really depends on how big a system you want. I have a 500l system myself, in my lwb transit connect. Most days i could manage with 350l, so if you plan to do trad as well as wfp, that should be plenty. For a 350l system, you want to be thinking about a payload of 700kg+. So current shape doblo (750kg), higher payload versions of the berlingo/partner (850kg), higher payload versions of the transit connect (i.e, swb 220, and lwb 230, which are 825kg and 900kg).
If you want a bigger tank, my connect manages (just) with 500l, so you want to be looking at 900kg+, and so on.
Good luck with getting a decent van though, its a minefield out there!