petcos get their cresteds in at a very young age, unsexed. what is likely the case in your area is that there is not a high demand for them and they are sitting in the store not being sold until they grow sexually mature. after that, people are probably going to snatch up any females, and you are left with a high number of males, so no, it's not uncommon to find them. it will be harder for you to find a sexed female unless you want to spend $100-200.
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4.1 snakes, 4.6.1 crested geckos, 0.1 gargoyle gecko, 0.0.2 red-eyed tree frogs, 2.0 devon rex cats, 1.0 betta, 0.1 sun conure, 1.0 lovebird