Looking through some old notes...
May not be demon-specific, but I found some magic items and spells I had listed out perhaps a decade ago. It's hardly inclusive, perhaps only covering products from the first year or two of Planescape, but here it is:
Spells:
* Campaign Setting:
Surelock, 4th level priest spell
Warp Sense, 2nd
* Planes of Law:
Spirit Wrack, 6th
Cacofiend, 7th
True Name, 7th
and something about Archon priest spells...
Gate Ward, 2nd
Gate Seal, 3rd
Cubehopper, 6th
Magic Items:
Lightbringer (minor)
Planar Mancatcher
Scourge of Lightning
Scythe/Sickle of Light
Scythe/Sickle of Pain
Scythe/Sickle of Paralyzation
Planar Wards
Shifter's Manacles
* In the Cage:
Chronosphere of Lagos, p39
Helm of the Dabus, p41
Lantern Staff, p16
* Player's Primer to the Outlands:
Mimir
* Planes of Law:
Chains of Light (Arcadia p7)
Vambraces of Evil's Warding
* Deva Spark (the Items of Accord):
Chalice of Peace
Waters of Serenity
Gem of Harmony