One could consider axolotls to be a kind of neotenic tiger sal, but I suppose you don't mean those. Do you also not mean Ambystoma andersoni? Those were available in the past and will probably be available again at some point.
When people find neotenic tiger sals in the wild and bring them into captivity, they rarely remain neotenic for very long. The stress/changes of captive life tend to induce metamorphosis. So the critter that you are probably alluding to really don't exist in captivity, but it's not a matter of availability, it's just that they don't stay neotenic in captivity.