Ihad a hard time finding a set for my 64. It has 14's on it, but I wanted original 13's. I finally found some by fluke after years of searching. My neighbor had a large pile tires and rims on his farm. He started cleaning them and low and behold he had 4 from a old Econoline shop truck hwe worked at before retiring.
Another friend here also lucked out. He saw a parked Mercury Econline van in a feild. Made a deal to buy it from the farmer. Inside were 5 13" rims with Mercury moon hubcaps. He been looking also for a few years. They are out there, but you have to search for them. The other thing is the early Valiants had 13" rims and they are just a bit different design.
The other thing me and my friend found was the Falcon/Comet rims in 13" are narrower, where the van ones are wider. I think the Falcon ones are 5" wide or 4.5" wide. The vans are 6" I beleive.