Best equipment with gold sword in some sidecave near ice dungeon (on mainland, not the volcano in the northeast island) and a lightning magic staff in the water dungeon (in the center of the mainland, building surrounded by a small lake on map).
For other equipment:
Green Armor: Default from beginning given by goddess.
Elf clothes: Some chest in elf dungeon (somewhere northwest at the sacred tree).
Handcuffs: Lose to bandits in some south sandy area near sandy dungeon (south-west). Unequip by talking to a guard.
Blue Armor: Obtained somewhere late-game. Either some chest or in some store.
Red Armor: Obtained somewhere late-game. Either some chest or in some store.
Wooden sword: West-most town, arena in north. Need to use this to battle in arena once.
Normal sword: Default from beginning given by goddess.
Gold sword: Above. (Sidecave near ice dungeon).
Rainbow sword: I got this at the end of the game, but in the starting town northeast, there's a building with a girl inside, and downstairs the sword is in one of the chests. Got a key from old man in north building (one of the starting locations) and paying 2,000,000. This sword uses magic and fires 3 rays when slashing. Can't use without magic, so it sucks.
Ice magic staff: Some dungeon. Probably some ice dungeon (either north-most or north-east island).
Lightning magic staff: Above. (water dungeon in center).
Tentacle staff: Same as rainbow sword. Powerful, but needs Tentacle underwear equiped to equip.
Barrier staff: Can't remember. Somewhere endgame-wise.
Weak Flame magic book: Bought in elf village shop.
Weak Ice magic book: Bought in elf village shop.
Weak Lightning magic book: Bought in elf village shop.
Slave collar: West-most town, arena in north. Lose at arena. Unequip via using an item from talking to some old man in a house somewhere east of starting town, sex scene, then buying for 160,000. Item can be used infinitely.
Magic collar: From old lady who introduces you to elf village, I think.
Blindfold: Can't remember.
Green collar: From some abandoned house. Not sure, but I think getting this requires letting go of some side-quest. Doesn't matter, I think.
Pink 1HP hard-mode collar: Endgame stuff, I think. Maybe chests in demon-king dungeon end.
Slave mark accessory: Same as slave collar, lose in arena. Funnily, if you skip the sex scene from being bought, you don't get this. Unequip by using item bought by witch on right (can't remember how much) in some witch house on east-side.
Witch mark accessory: Ask the witch on left and answer in some way to add it on. Unequip by using item via witch on right.
Lovejuice collection accessory: Bought somewhere. Forgot where.
Semen collection accessory: Bought somewhere. Forgot where.
Rotor accessory: Some town without much significance. South-right house, need to force into house, lose to guy, he will play with you with rotor and dildo and put you in handcuffs.
Dildo accessory: Same as rotor.
Tentacle underwear: In southwest sandy dungeon in blue chest (which I think blue chest means anything that is equipment, red for keys to unlock some dungeon doors, and brown for items).
There's probably a lot I'm missing (from side-quests and stuff I can't remember, going by memory here and don't want to open game again).