Anyone know where I can find a wheel arch replacement panel for an 03 Ram?

Good luck finding parts for the truck, since it was done for 3 years only (03-05). Your best bet is going to be (1) pre-lube than go to the dealership, (2) call companies like LKQ who specialize in buying wrecked vehicles than supplying body shops with the parts. Should be the cheapest way to go.

http://www.lkqcorp.com/search-inventory.aspx