Your local auto wreckers would be the 1st place, and they have access to the large network of all the other auto wreckers/recyclers.
Sometimes you can find a local truck being parted out, but that’s far and few in between.
You can also look at car-parts.com. They have listings for many yards. Swapping cabs will also require getting the VIN updated for your truck. The VIN of course is visible under the windsheild. If it doesn't match your title/registration, it can be a lot of legal issues. Moving VIN tags is highly frowned upon typically.