Don't look for a chicken vet, look for a vet that frequently deals with avian exotics.
I would ask the seller about the chick- ask if it is one of the bonus ones.
It could be an error, or an injury,. sometimes things don't go as planned despite the best intentions
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To me, it looks like a gross deformity but I could be wrong. Your best bet to figure out if it is a birth defect or a bonus chick that might not be such a hot bonus is to ask the person you got them from. If it is a birth defect, I am guessing it is too late to fix. Also, it looks as though the joint might be involved? Can the chick bend the leg properly and does it have proper range of motion (at the knee, at the hip joint, at the ankle)? I had a couple chicks hatch last year that I had to euthanize because they had joint deformities that, unlike spraddle leg, were not fixable and would have permanently affected the chicks quality of life. Basically, the knee didn't bend the right way, and they couldn't ambulate well.
So, ask the seller about the history, even if you do take the chick to a vet the vet will want to know as much about the duration and history of the problem as possible.