I have only ever seen these in pet stores time to time. It probably be a good idea to check your local chain and independent stores. Ask if they can order them in. Their animal suppliers may carry them but it may not be something they bring in as there isn't much demand, but they could special order some for you.

The fact they are not really available can also be a good indication they don't sell well as pets compared to normal mice, so that is something to consider, if you are just breeding them to sell as pets.
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PHLdyPayne