This is probably the reason for all the fleet vehicles available in auctions that have no catalytic converter because theirs was stolen. The municipality or company that owned them found out that replacements would be nearly impossible to get so they opted to sell at a loss to someone who could afford to wait or who operated in a different state where OE replacement catalytic converters are okay for use. I believe that California and Colorado have banned converters that do not come from the manufacturer, effectively preventing anyone who had theirs stolen from using the vehicle legally even though aftermarket units are available that meet 50-state requirements.

Good luck. It may be an option for you to sell the E450 to an out-of-state buyer and replace it with a used or new vehicle. I don't know whether you can make that work for yourself.