NAPA sells tar-paper like adhesive sound deadening insulation, although this might be the most expensive route you can go considering the cost/vs size of adhesive sheets. Another option is go to home depot and get some heavy rubber mats cut to cover the floor area. sound deadening is a tricky art- there is a reason that a lexus and a mercedes are quiet, which has to do with how the car is engineered from the ground up to deaden sound and be quiet, something that is not even added into the cost equation for something as utilitarian and realtively "cheap" as a service van. There is only a certain amount of deadening that will even be possible due to the design of the vehicle. Not to discourage you, but just something to keep in mind. Sound waves transfer best through substances that are dense and uniform, and transfer worst though substances that are amorphous, which is why I sugest heavy rubber mats. Sound deadening materials used in car wheel wells etc. are tar or asphalt like for this reason- on top of being cheap they are molecularlly amorphous substances that tend to absorb and dissipate sound waves rather than transmit them. Same is true of rubber mats..
