Junkyards would be your best bet. Thats where i got mine for my ST-SLT conversion. If you can find a U Pull or Pull A Part type place, there are often plenty still in the trucks. Most cost ~$50 at a you pull it yourself place. Speedos from 02-05 will fit. Not sure about 06+ in an 05. Be sure you get one with the RKE keyless entry box on the back if your truck is equipped with it currently. The milage will read the same as the truck you pulled it from unless you can find a company to edit it for you, they’re around. It is also a federal offense to sell a vehicle with a modified odometer (Not sure if disclosing the change covers your rear). You may need to have your keyless entry reprogrammed to the new speedo if swapping the module from the back of yours to the new one doesn’t work.
Good luck.
Edit: Also be sure the speedo is from the same drive train as your vehicle, Auto or Manual, and Diesels have diferent displays.