I bought a JR Products city water inlet recently and it didn't come with a washer of any sort.
The interior side of an inlet gets connected with a "swivel" fitting (usually plastic in an RV) that has a seal in it like in the photo. These seals can occasionally fall out when handling the fitting. If that's your "washer" that is missing, I don't think you can get just the seal. It's not a part related to the inlet. A complete swivel fitting is around $5 or less and not worth the cost of shopping or driving all over to find a seal. Once the fitting is installed, you shouldn't need a spare because you'd rarely ever disconnect the swivel fitting. Many RV dealers stock the swivel fitting, and they're at Home Depot and plumbing wholesalers and hardware stores.
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