If you're good at juggling, go to Neverland. The Rare Truffles can give you 3 per Truffle. So 12 per turn if you're good at it.
But if you're like me and not good at it, EotW is a very good place. Either you get White Mushrooms (with all possible magic options however) or you get enemies to farm other materials which you'd also need lots of.
Another way I found is this, for both Black Truffles with Mystery Goo and Pot Spiders for Mythril Shards:
Equip Enounter Plus, Guard and Ripple Dive. Switch from the Bazaar to the Palace Gates constantly. This way you'll encounter the Pot Spiders, Pot Scorpion and Black Truffles.
The Pot Scorpion is obvious... break all pots first before killing it..
The Bazaar area is strategic though. If you get regular pot spiders, summon Bambi and just use Thundara on everything. Though even with all 3 Lucky Strikes you'll get only 1, maybe 2 Mythril Shards per time. Plus a Blaze Gem usually too which might be convenient.
If you get the 4 Black Truffles, summon Bambi also. By the time he's summoned, one of the Truffles has fallen down on your platform. Immediately cast Stop(ra/ga) on it and give it a combo*. After this one is dead, the other will drop down on you for the same process. Immediately drop down right after and you'll find the last two Truffles in a corner "trying" to get to you. Simply cast Stop(ra/ga) again and give them both a combo. Ripple Dive helps a lot here since this finisher has a wide enough range for both.
Xelon, that's good advice but I'm afraid they're playing Vanilla KH1. That means no Pot Scorpions, Ripple Drive or Encounter Plus. Most of the rest of the advice should be good. End of the World is a great place to hunt for materials, unless you're trying to get the Arts items from White Mushrooms.
One thing I like to do to get Mystery Goos is to fight Black Fungi in a different way. Black Fungi will drop Mystery Goos if you kill them with a "Critical Hit," which is how KH1 refers to the final hit of your combo. But it's not worth it. Instead, hit them with a special attack, which counts as a Critical Hit.
Trinity Limit is good, because it can kill more than one fungus at once, but it can only be used if Donald and Goofy are alive and Black Funguses are good at killing them. And yes, the Fungi often go invincible during the long casting animation. But if you kill even a single Black Fungus with Trinity Limit, you'll get a single Mystery Goo, so with luck and a few ethers it isn't hard to get two or three.
You can also use other special attacks. I've seen a post saying Ars Arcanum is good. This would save you from having to worry about Donald and Goofy, Trinity Limit's slow timing and high MP cost, but Trinity Limit worked fine for me and I never tried any others.
Black Fungi also have a 20% Mystery Goo drop rate if you kill them normally, so you have a 1/5 chance of getting what you want no matter how they die.
If you've never used a special attack before, it's kind of weird: equip the special attack ability, and I recommend you unequip all other special attack abilities. Then, when you get into the right situation, it will appear at the bottom slot of your command menu. For Trinity Limit, it should be there most of the time in combat, so long as Donald and Goofy are alive and you're standing on the ground. For Ars Arcanum, it only works if you
and the enemy are on the ground.
