I've used the little hopper mice from Petsmart, and I think that they are a pretty good product. If you are getting a small kingsnake, they might be fine for your snake's entire life. They are a little small for my California Kingsnake, but I sometimes give him one as an appetizer.
People talk about the economy of ordering online, and I agree that you can get them cheaper per mouse when you do so. On the other hand, if you have only one snake and feed only once a week, roughly $2 per mouse still only adds to $100 per year. If you're making your purchase at Petsmart, you can buy these mice in a pack of six every six weeks. If you order online, you'll be getting probably 100 at a time. If they are vacuum sealed in packages of ten or twenty, they'll be fresh for a while. They may even stay fresh for two years. If they are all in one package that won't be vacuum sealed after you've opened it to get the first mouse, they will lose freshness in the two years that you'll need to use all 100 of them. If you know someone else who keeps snakes and who can split an order with you, ordering online could be worthwhile. If you are just feeding one snake and aren't ordering with someone else, you might find it easier to get your mice from Petsmart or some other place that sells them in small quantities.
Bill
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It's not how many snakes you have. It's how happy and healthy you can keep them.