I hope you are a very good sewer because if did buy store bought then it would probably be very expensive.
Get blue stage makeup for the face and the tail instead of dragging on ground or stiff that would poke someone's eye, the end of the tail can be tied to back of neck collar so it's always up.
patterns = blurayrippers.net/iyXaf5t/
store bought = avatarcostumeideas.com/
Avatar Neytiri Child Costume = $30