In a brochure or on a web site you might find the range of offered GVWRs for a particular model, or you might find the maximum offering, for which there might be very few trucks.
GVWRs are determined by options: GVWR package options, "heavy duty" or "high payload" options, and wheel and tire choices. Sometimes some of these options also get tied to trim levels or trim packages.
To order a new truck built (or to be searched from inventory) you negotiate options with the salesman. Actually, you try to negotiate with the fleet sales manager, because most sales people I encounter don't know that much about the vehicles they are selling.
To find the GVWR of a truck on the dealer's lot, you open the door and look at the DOT weight sticker. If you don't like the number, maybe the salesman can point you toward an example with the options and trim packages that make GVWR higher, like uglier smaller wheels with higher profile tires.
The real problem is going to be how you find out what the vehicle actually weighs, and how much of that GVWR is left over to carry people and cargo.
My suggestion shopping the lot is to look for the 250/2500 or 350/3500 series in "work truck" trim with no more seating than you actually need, and as few weight-adding options as possible. Some dealers keep a few in stock, salesmen don't like to sell them. But most of them get bought buy companies who want to carry the most, or pull the most, at the lowest cost. If buying more than one at a time, they'll usually go to the fleet manager to find the trucks.