Kingdom Hearts

From the Item Synthesis FAQ:

Mythril Shard
A mineral shard containing rare metal.
- Barrel Spider (Neverland)
- Behemoth (End of the World)
- Pot Spider (Agrabah)

The barrel spider and pot spider drops them at a rate of 1%. Just equip your party members with lucky strike and you should get 1 in less than 5 mins. Try the bazaar in Agrabah. Everytime you enter the room, about 14 pot spiders will spawn

Pot Spiders and Barrel Spiders drop rate for Mythril Shards is 1%, those guys wouldn't be a good source for Mythril Shards. Behemoths' drop rate for Mythril Shards is 20%, with Lucky Strike equipped on everybody it will probably increase to 45%. Behemoths are located at End of the World. Go to the area where there are floating platforms that points at each other and once you reach the end you will encounter a Behemoth. This is a good way to get Orichalcums and Mythrils also!

If you're still confused, you should look up an FAQ on this.

It should be in the Hints and Cheats section.

It'll go into detail on what you have to through.

If you press ctrl + F and put in what you're looking for you'll find it faster.

Hope this helps. :D