Bahamut Card - FF8 Guide
Location: Deep Sea Research Center
Card Mod: 100 Megalixirs
First Available: Disc 3
The Bahamut Card is obtained by defeating Bahamut inside of the Deep Sea Research Center which you can first access after acquiring the Ragnarok on Disc 3. Bahamut is the first boss that you will face off against in this dungeon, he's literally in the very first room of the dungeon.
On Disc 4 if you completed the Card Club Side Quest you'll be able to play members of the CC Group on the Ragnarok and win this card as well as many other Rare Cards from them. This quest must be completed prior to the end of Disc 3 for you to take advantage of it during Disc 4.
You can Card Mod the Bahamut Card into 100 Megalixirs which will solve basically all of your group healing needs for the rest of the game. There are plenty of other ways for you to obtain Megalixir items, especially if you use the Esthar Unlimited Money Trick... However this way is by far the fastest.