There are only a few of these very handy materias to be found. The first one you get is at Shinra Headquarters after you rescue Red XIII. There are also Enemy Skills materias at Junon, the Forgotten City, and the Chocobo Sage's House.
The best way to get enemy skills from enemies is to manipulate them, then have them cast the desired spell on your people.
Level 4 Suicide:
Find: Grasslands Area, Mu
Does heavy-duty damage to all targets whose level is a multiple of 4.
Find: Grasslands Area, Custom Sweeper; Corel Prison, Bull Motor; various others
Does light type-less damage to all enemies.
Find: Corel Prison, Death Claw
Reduces a target's HP by 1/2. Doesn't always work.
Find: Gongaga Area, Touch Me; Temple of the Ancients, Toxic Frog
Casts Sleepel and Toad on one target, or cures Toad from one target.
Find: Junon Area, Zemzelett; Whirlwind Maze, Wind Wing
Refills the HP of all allies by an amount equal to the caster's current HP. Also relieves most conditions.
Find: Corel Area (shoreline only, near Costa del Sol), Beach Plug
This handy Enemy Skill casts Barrier, MBarrier, and Haste on all targets.
Find: Wutai Area, Razor Weeds
Steals 100 MP from a single target.
Find: Wutai Area, Adamantaimai
Protects the targets from instant death spells like Death.
Find: Mythril Mine, Archdragon; Nibel Mtn., Dragon; Northern Crater, Dark Dragon.
Does decent Fire-typed damage to one target.
Find: Gold Saucer Area, Harpy (desert only); City of the Ancients, Jenova LIFE.
Does decent Water-typed damage to all enemies.
Find: Grasslands Area (swamp), Midgar Zolom
Does moderate to heavy Fire damage to all enemies. Midgar Zolom will cast this one only if his health is very low, and you can't manipulate him.
Find: Gaea Cliff, Molbor; Nothern Crater, Marlboro. (I find it very amusing that Marlboros give you "Bad Breath.")
Attempts to cast Poison, Sleepel, Mini, Toad, and Confu on all enemies.
Find: Nibel Mtn., Materia Keeper; Gaea's Cliff, Stilv
Does moderate Lightning-type damage to all enemies.
Find: Gaea's Cliff, Stilv
Does pretty heavy Fire, Ice, and Lightning damage to all enemies.
Find: Shinra Mansion, Jersey; Midgar Sector 8, Behemoth
Does type-less damage equal to the caster's max HP minus his/her current HP to one target.
Find: Grasslands Area, Level 16 Chocobo
The way to get this is to buy 3 Mimmet Greens at the Chocobo Ranch, then go right outside and look for a chocobo that is accompanied by either two Elfadunks or two Levikrons. Give the chocobo all three Mimmet Greens, then hit him with a Level 4 Suicide. He'll get so mad that he'll cast Chocobuckle on you, which does damage equal to the number of times you've run away from battle.
Find: Goblin Island, Goblin
Does very little damage to one enemy, unless your level is the same as the enemy's (a pretty rare event), in which case, it does very heavy type-less damage.
Find: Junon/Cosmo Area, Ultima Weapon; Northern Crater, Dragon Zombie
One enemy is hit for very heavy, type-less damage.
Find: Final Dungeon, Dragon Zombie
All enemies are hit for heavy type-less damage (and it looks cool, too).
Find: Final Dungeon, Pollensalita
Completely refills the HP and cures conditions from a single target. Will also revive and refill a dead character's HP. Probably one of the best Enemy Skills.
Find: Final Dungeon, Dark Dragon
One target's physical and magic defense are increased by 50%.
Level 5 Death:
Find: Final Dungeon, Parasite
Instantly kills all enemies whose level is a multiple of 5. Does not work on bosses.
Find: Cave of the Gi, Sneaky Step & Death Spector
Condemns one enemy. When his counter runs out, he's toast.
Find: Final Dungeon, Death Dealer
Probably the most elusive of Enemy Skills. This one starts a little game of roulette, where one person in the battle, enemy or ally, is killed instantly. Doesn't work on bosses, so don't even try.