They can be sold for 100 gold at Medium size and for 300 gold at Enormous size. Every level above 1 additionally increases their price by 5%.
Saw a wild one for the first time in a raining water portal. Don't know which one's the requirement, but it looks like it needs to be wet. ← Rain makes water spirits randomly spawn as enemies. Elexolotls can spawn in the place of any fully randomly generated spirit (ie. not from a biome's specific nonhumanoid table), so spirit weather (of any kind, but blizzards and heavy rain have the highest spawn chances for spirits iirc) is a good way to find one. They also have a vastly increased chance of appearing exactly on day 0.