I am sure you are finding that diesel fuel caps are easy to find and gasoline fuel caps are very difficult to find.
You will need to determine if you need female or male threads and the diameter required (ID for male and OD for female).
Once you have that you should be able to find what you need at any parts house that deals with medium and heavy duty parts.
If not you may find it necessary to start searching bone yards.
You might try Neal's Truck Parts in Grand Rapids, MI. Neal's Truck Parts
Or Anderson Brothers Truck and Equipment Co. in Eugene, OR. Used Trucks for Sale, Truck Equipment for Sale. Trailers, Freightliner, Ford, International, Peterbilt, Kenworth