The first Enemy Skill Materia to be found in “Final Fantasy VII” is after a boss fight involving the scientist Hojo. After the fight, the Enemy Skill Materia will drop in the containment chamber that the experiment rose up in. The second Enemy Skill Materia is then found in an inn located within Junon. A third Enemy Skill Materia can be found towards the end of disk 1 in one of the abandoned shell-shaped houses in the City of the Ancients.
All three Enemy Skill Materia can be found within the first disk, however a fourth can also be found quite a bit later on in the game, after obtaining the airship Highwind. Once the aircraft is obtained and the party is able to fly across the world, deep in the snowy mountains, surrounded by sheer cliffs, tucked away in a small corner of rock, is the house of the Chocobo Sage.
This Chocobo Sage provides info regarding Chocobo breeding and the like, and if one speaks to the sage's Chocobo, kept in a small pen inside of his house, the player will be rewarded with the fourth Enemy Skill Materia. Due to the fact that certain opportunities to obtain specific enemy skills may have been completed by this point, the fourth Enemy Skill Materia is the hardest to fully obtain every enemy skill on, perhaps even impossible, depending on the time of aquasition and the player's actions up to that point.
For example, the enemy skill Trine can only be obtained three times throughout the entire game. The first time being in Mt. Nibel against the scorpion boss guarding the exit, Materia Keeper. Due to this being rather early on, it is only possible to learn the skill on two Enemy Skill Materias.