Best thing to do is check the kingsnake classifieds for adult dragons. Most may be hold backs from last year or retired breeders. Though some may be thinning out or getting rid of their collection. Even if you don't see any adults for sale, emailing or calling breeders and asking if they have any adults for sale, may reveal positive results. Some may have older dragons but may not bother advertising as most people don't buy adult dragons, perferring to start with babies.
There are plenty of good bearded dragon breeders out there. All should be able to give you indepth details about the health, size, age, friendliness of the adult dragon(s) they have available to sell. If they cannot or will not provide current pictures, answer all your questions etc, then I would suggest looking elsewhere. The only acceptions would be rescued dragons who's history/genetics may not be known.
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PHLdyPayne