Kingdom Hearts II Probably a noob question but eh... ^^;

Umm I forget. Where can I get Mega Elixirs? or Elixirs?

 

In KH:CoM, you first get an Elixir over in the Hundred Acre Wood. It's presented by Roo when reunited with Pooh. Megalixir is gained at Destiny Islands in the Key to Rewards room.

In KH2, you can synthesize an Elixir using Mega-Recipe + Mythril Stone x3 + Power Stone x1 and Dark Stone x1. You can also get them from Devastators (5% chance) or Living Bones (5% chance). They're also chests in Crystal Fissure and the Mansion Foyer. Synthesis Rank: A & Synthesis EXP: 15.

For Megalixir, you need Star Recipe + Mythril Stone x3 + Power Stone x1 + Dark Stone x1 + Serenity Gem x1. You need to obtain 15 Synthesis Material Types. Synthesis Rank: A & Synthesis EXP: 15.

Don't know about KH1, though...

 

Thought I'd finish off the answer since Destiny's Force explained KH2 and CoM so here's KH.

Megalixers in KH

Megalixers are synthesized by using 5 Bright Shards, 3 Bright Gems, 1 Bright Crystal, 1 Shiny Crystal and 2 Gales.

In Wonderland - you can obtain one if you go to the rabbit hole and fight some enemies after defeating the Crank Tower. The first time you fight them a chime will sound and a treasure box with a mega-potion appears. Then return to Wonderland after Hollow Bastion is defeated and fight the enemies again to get a chime and another box with a Megalixir in it.
You can also find 1 if you set the clock tower at Neverland to 12 o'clock.
There's a megalixir in Agrabah at the same room as Dalmations 37,38,39 and there's a Save spot there.
There are 2 megalixirs in Monstro.
There is a megalixir in Hollow Bastion on one of the ice platforms you jump on at the beginning of the world.
At the End of the World there are 5 megalixirs about. There are two pretty well at the beginning of it as you travel through the clouds area. Another one is the machine in the lab, there is a megalixir there in that room. The last one is in the area before going to Traverse Town around the pink door.

Elixirs in KH

Synthesis: 2 Blaze shards, 2 Frost Shards, 2 Thunder Shards and 4 Mythril Shards.
Halloween Town - in the gray pumpkin's mouth there is an elixir in one of the boxes. In the other box is Dalmatians 71,72,73.
There's an elixir in that Cave of Wonders first area I think. There's another one in the room past that where the giant balls roll around on a ledge area.
There are 3 elixirs in Atlantica. Two I am sure are in the area where you swim out past the save point (not the one in King Triton's area).
There is one elixir in Hollow Bastion and there are quite a few other things in the same room which is the library.
Some Heartless such as the Battleship, Defender, Fat Bandit (after Hollow Bastion), Invisible, Rare Truffle (juggling him 10 times), Shadow Sora and Wyvern all have a percentage chance of giving up elixirs.