I have found from most of the vans iv'e owned that either ford 4 stud or vauxhall usually fit but look around the scrap yards after mesuring the van wheel as danny says measure the distance across the studs and the size of the centre hole and check the offset by placing a straight edge across the tyre and measure the gap to the centre of the rim and apart from the tyre size thats it
for years I never carried a spare van wheel as the car I towed with fit perfectly so I had a ready to use spare in the boot