We originally wanted an f150 EB, hd/tow package in a used truck. Looked for almost a year. Finally gave up and got a 5.4, which took us almost 6 months to find. It's a 2010 although we would have liked newer. Only had 50K miles, and has a payload of 1750. We had to give up nice-to-have options to get that payload. Years ago we searched for a Suburban 2500, 8.1 engine. Looked for 6 or 7 months for that also!
Good luck with your search. It just takes time when you're looking for something specific. We found ours at a Chevy truck dealer!