Welcome James (lugerson),
The belt should look similar to this one that was sold on eBay two years ago...
You may find similar buckles that have an alternate inscription but a similar crest. Mine has "G�¶tt Mit Uns" for the inscription.
If you find one suitable for your Dad's Luger rig, it will likely be not in a condition that would serve well for actually wearing it and carrying the pistol... it would be more for just collecting and display because leather doesn't age as well as gun metal.
If you think he might actually want to wear the belt for re-enactment purposes, I would recommend that you get a good buckle as seen in the photo above and then obtain one of the surplus East German police belts that are of similar construction and that are often found on eBay. The WW2 buckle should fit the later belt.
Good luck and let us know what you find and buy him...
If you have the ability, how about posting some good photos of your Dad's Luger and accessories?