If I've already played a Supporter this turn and use Ninetales' "Supernatural Shapeshifter" attack, can I still use the effect of a Supporter I discard from my deck?
Yes, you can. You are not playing a second supporter, but using its effect via an attack.
Source: Mega Evolution FAQ; TPCi Rules Team (2025-09-25)

Relevant cards

Note: All applicable cards may not be listed below.
ME1_20
ME1_20