Your best bet might be eBay. The steel ones pop up often, maybe every other month. I rarely see any that don't need repairs though. The NOS rust free ones that do show up on occation go for big $$$.
Old trucks, it's a sickness-one I hope they never find a cure for!
Bob Jones--Fat Fender Aficionado--FTE member since '96
49 F-2 pickup/48 F-1 panel truck
48/51 F-4 flatbed (2 in 1)/49 8N tractor