What type of Transit are you after?
Panel van? Minibus? Tourneo? Tipper? Dropside? Luton? Box van? Fridge van? Beavertail? Spec lift? Horse box? Burger van? Ice cream van? Mini artic? Chassis cab? Crew cab? SWB? MWB? LWB? Jumbo? low, medium, or high roof? Slow and cheap or fast but costs more? FWD? RWD?
There is a LOT of choice
Probably the best places to start looking are ebay, gumtree, auto trader, and of course this site