For the price of the conversion, you could buy two or three of my tow rigs...
...with that said, I would think anywhere that deals in semi tires would be able to help.
this is a dumb idea.
I'll have all the info today jeefypoo
you can get wheels that are your stock lug pattern that will take the OTR tires so you don't have to use adapters, but they are spendy.
http://www.ricksontruck.com/
http://www.ricksontruck.com/wheels_dodge_drw.html
$300 per wheel for steel and $500 per wheel for alum.
245/70/19.5 tires run $3-400 each
http://www.ricksontruck.com/tires_24570R195.html
HTH,