The economy is great and fuel prices are generally low. Expect to keep seeing what you’re seeing, a low inventory of newer used gas guzzlers.
Also, a much greater % of HD trucks are diesels. Most of the gasses are fleet vehicles. Bet 7 of 10 used trucks like you’re looking for are white!
Why 250? Because there are very few 350 gassers.
You are looking for a vehicle that there are not a lot of compared to diesels or half tons.
Just keep searching, or go buy a new one. $10k off the sticker on a low to moderate equipped truck isn’t much more than your budget.
Used truck market is hot still.