My memory is fading me but I'll try...
Mystery Goo can be obtained by any of the three funguses/mushrooms. You remember those White Mushrooms that you cast spells on and who'd give you items in return? Well, they'll give you Mystery Goo, but I don't really like them. Black Fungus will give you it, but rarely. But if you use Trinity Limit on them, you have a 30% chance of getting a piece of Mystery Goo. And then there are the Truffels, my favorite way of getting them. Every time you hit them, they go flying in the air. Hit them a certain number of times, and they'll drop items. Hitting them is all about getting a good rhythm with your jumping and attacks, which was easy for me. All these heartless appear randomly, but I found that you can find all three quite often in the room right before the final save point. I haven't really played KH at all recently, so pardon if any of my information is incorrect. If nothing else, do a search on GameFaqs.