There's a shop in Mechanicsville that has them but they are really high maintenance animals that need very particular cold water conditions. They also shouldn't be touched unless absolutely necessary which might not make them the best for a kid (their skin is made of cartilage, very sensitive, and permeable).

They're really cute but I'm tired of seeing my friends buy them and then watch them slowly die. They're amazing creatures and I highly encourage getting them as long as you're willing to fully commit to their care. Sorry if this comment is negative.

Might have better luck checking DC and making a day trip