You could look at the Kayak Car Search. I tried for a few European combinations and it came up with various van and van-like options in it's 'Car Types' filter. Although, to be fair, the options seem to be limited in some places.
Helpfully they offer a list of other sites and invite you to compare Kayak against them.
Priceline, Orbitz, Travelocity, AutoEurope and CarRentals.com all allow you to filter on type and find a few large vans and some smaller passenger vans / SUVs.
Note, I think you want a 'real' van rather than what those sites might be calling a 'van' which is more an SUV. But some of them did match genuine vans of the type you may be looking for.