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

The interchange for that bed shows 2002 through 2009. If you want OEM metal try www.car-part.com. Look up the bed side for the oppisite side and then call them to see if the section you need is still good on the side they don't list. If a bed side is too damaged they won't inventory it and that is where I locate patch panels. Search by your ZIP code and distance so it will show what is closest to you.