i bought generic mudflaps from advanced auto parts for $10

 

Polyurethane is exactly what you need and it isn't cheap. I also went DIY, bt the only reason I did it was to go a little bigger than the rallyarmor ones. Mine are slightly the same just 2" longer and 1" wider for more coverage. I would say either spend the money on a good set from rally armor or just go to an autoparts store and get some generic ones and make them fit. Here is a pic of the ones I built. I have about 60 bucks just in material. Good Luck!

Sorry for the cel phone pic