Spell Focus

Spell Focus item enchantment
Spell Focus item enchantments, including the Spell Focus Mastery, also increase DC of the spell and stack with the feat.

+1 versions
Can be added to items with Cannith Crafting.

Can be found on randomly-generated weapons, bracers (and not rings) and various named items. Can also be slotted in  and such on epic items.
 * Abjuration Focus
 * Conjuration Focus
 * (Divination Focus doesn't exist)
 * Enchantment Focus
 * Evocation Focus
 * Illusion Focus
 * Necromancy Focus
 * Transmutation Focus

Minimum level:
 * Weapons (scepters and such)
 * level 4+
 * level 8+ (foci in two schools, in one item)
 * Bracers
 * level 7+
 * level 11+ (foci in two schools, in one item)

+2 versions (Greater Focus)
Can be added to items with Cannith Crafting.

Greater Evocation, Transmutation, Conjuration, and Enchantment Focus can be found on the third tier of Alchemical Crafting.

Also found on various named items:
 * Greater Enchantment Focus
 * Dreamspitter
 * Epic Elder's Cap
 * Epic Spectacles of Spirit Sight
 * Scepter of Mad Trickery
 * Shard of Xoriat
 * Staff of the Petitioner
 * Xachosian Eardweller
 * Greater Evocation Focus
 * Belashyrra's Scepter
 * Black Dragonscale Robe
 * Bracelet of Madness
 * Upgraded Deific Diadem
 * Epic Robe of Fire
 * Epic Shaman's Beads
 * Regalia of the Phoenix (unsuppressed)
 * Greater Illusion Focus
 * Epic Illusionist's Garb
 * Scepter of Mad Trickery
 * Greater Necromancy Focus
 * Belashyrra's Scepter
 * Death's Touch (upgraded)
 * Epic Docent of Shadow
 * Epic Robe of Shadow
 * Epic Spectacles of Spirit Sight
 * Staff of the Petitioner
 * Greater Transmutation Focus
 * Scepter of the Fleshweaver
 * Master Transmuter's Staff

+3 versions (Epic Focus)
Can only be found on epic items.
 * Epic Conjuration Focus
 * Epic Diabolist's Docent
 * Epic Diabolist's Robe