Extra body slot (Dragon 359, p117) directly implants an item into your body. The down side is, it permanently reduces your con score by 2, and you need a specially-crafted version of the item that costs 1.5 times as much as normal.
The eldritch haunt template (Towers of High Sorcery, p149) can create a creature with the properties of a magic item, but it's not available to player characters.
Adamantine skin (Faiths of Eberron, p157) or combat mod (Dragon 359, p116) are both ways of adding built-in armor to a character. Both allow the armor to be magically enhanced later. You can get a wide variety of magical effects on armor.