A butter is for sure a captive bred animal, and you are very wise to insist on captive bred. If the pet shop is reputable, if they can answer all of your questions, and the animal is healthy and kept under the right conditions, then of course, it is fine to buy from them.

I got my ball from a breeder, just because I personally wanted to buy from someone who specializes in snakes rather than a pet shop that dabbles in everything. That is a personal decision, not a hard fast rule. It is up to you.

If you do want a breeder recommendation, I got mine from BHB Enterprises
http://www.bhbreptiles.com/

I really like them and my snake is perfectly healthy and was shipped fast and correctly. There are countless other breeders out there that would be just fine as well.

However, I know that once you see an animal through the window with your own eyes, the attachment can be instantaneous. It sounds like you already have your heart set on the butter, and if it is healthy and well cared for, no reason why it shouldn't be yours!