Power Plus appears only in the 2D versions. It is found in the Earth Cave B4, optional dungeons, Whisperwind Cove and Lifespring Grotto, and can be dropped from Hyenadon. It will permanently raise Strength of one character by 1 - 3 points.
Power Sources permanently raise the Strength stat by one. It can be morphed from Unknown 1, Heavy Tank, and Screamer. It can be dropped by Helletic Hojo and found in Junon, Costa del Sol, Mount Corel, Rocket Town, Wutai Village (Turtle Paradise prize), Coral Valley, Midgar Raid, and Northern Cave.
Str Up can be mugged only from Raijin at Balamb. It can be obtained through refining a Monk's Code for a Str Up. 10 Hyper Wrists refine into a Str Up with the Guardian Force Doomtrain's Forbid Med-RF ability. A Str Up can also be found on the 2nd floor in D-District Prison.
This item is called Power and it has a potion icon before its name. It can be found in assorted treasure chests throughout the various worlds and increased a character's Str stat by 3. It's also a one time use item as opposed to other consumable items which have more uses. Only usable on humans or mutants.
The Attack item is a Robotic Parts that gives a permanent stat increase to Attack by 3, but it can only be applied when the character is a robot. If said character changes forms, they'll lose the stats gain through this item. It can be bought for 1500 GP in Cirrus, Dharm (Present), Donmac, Elan (Past and Future), and Knaya.