How do you get the Mystery goo in Kingdom Hearts 1?

It comes from a White Mushroom. Look for them in the Camp section of Deep Jungle. You get it from them after the third spell. This item is very rare, so good luck.

Here is a website if you need help finding any other items: http://www.kh2.co.uk/pages/kh/walkthrough.txt

another thing, be sure to try and get lucky strike it helps a lot

When you find the white mushrooms they are hard to beat. you have to use magic. each movement form the mushroom calls for a different spell.

When he Spins ues Aero, when he Stops moving use Stop, when he Shivers ues Fire, when he Fans himself use Blizzard, when he Floats use Gravity, when he Falls over use Cure target, when a Light appears over Mushroom's head use Thunder.

HOPE THIS HELPS