You will most commonly collect blaze gems from the Bandit and Fat Bandit, which are found in Agrabah. They seem to drop them pretty often.
However, you can also get them from the White Mushroom if Fire was the last spell cast on it, although they'll usually drop a blaze shard instead, so the Agrabah strategy is probably the best way to get them.
If you are having trouble collecting them, equip Lucky Strike on your party to increase the chances of the Heartless dropping valuable items.
Good Luck!
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Probably Agrabah
You find all your answers for Kingdom Hearts on "wiki Kingdom Hearts"
You can find kingdom hearts stuff at Hot Topic. They might be there. That would my first guess on finding them. They also have Girr stiff to for more info.
There are many places where one can find a Kingdom Hearts walktrough. It would be best to walk through the Blizzard first into Wonderland and proceed from there.
Youtube would be the best guess. It has millions of videos. Another option would be to find the official website for Kingdom Hearts 2.
Kingdom Hearts- Air Pirate, Battleship, Pirate Kingdom Hearts II- Aerial Knocker, Shaman.
There are no Gameshark codes for Kingdom Hearts 2.
The Heartless? You would find those in Halloween Town
Hot Topic, You can go on-line at hottopic.com and search kingdom hearts and there's a kingdom hearts necklaces and other things.
You have to earn them.
There is no possible way of finding Xemnas in kingdom hearts. You are only able of finding him in kingdom hearts 2, which he is the last boss in the game.
The best place would be The World That Never Was.