I want to do a 1 way car hire from Anchorage, Alaska and drop off in Skagway, Alaska but am having trouble finding a company that offers this.
One way rentals are few and far between in and around Alaska. The demand for cars is fairly regional (ie fly into Anchorage, drive around, fly out from Anchorage). There are rather few travelers who do something like, fly into Anchorage, drive around, then take the ferry from Skagway.
During the peak summer months, you may get lucky with one ways, as the car rental firms want cars in Skagway for renting to cruise ship passengers. But there will likely be a one way surcharge. You may find the reverse (Skagway to ANC) at the end of summer when they send the cars back. Outside of cruise season this is unlikely.
You may find it easier to rent the car in Anchorage, drive around sightseeing, return the car to Anchorage, then fly to Juneau and pick up the ferry.