Greater Spell Focus (feat +1 to a particular school); requires and stacks with Spell Focus in the same school.
Epic Spell Focus (feat +1 to a particular school); requires Greater Spell Focus feat of the same spell school and Lv 11 Cleric/Druid/Wizard, or Lv 12 Sorcerer/Favored Soul, or Lv 15 Artificer/Alchemist, or Lv 16 Bard
Spellsinger enhancements, +5 Enchantment, Evocation, and Illusion from spellsinger core, universal +1 from Spell song trance, +2 Enchantment from Prodigy, +1 Enchantment, Evocation, OR Illusion from Marigold Crown
Spell Focus item enchantments, including the Spell Focus Mastery, also increase DC of the spell. They are Equipment bonuses, and thus stack with the feat.
On randomly generated items, school-specific Spell Focus is available up to +6 and Insightful +3.
On crafted items and set bonuses
You can use Cannith Crafting to craft school-specific Spell Focus +1 to +6 and Insightful +1 to +3.
You can use Cannith Crafting to craft universal Spell Focus Mastery +1 to +5
Alchemical Crafting (tier three) offers +2 bonus to selected schools (Evocation, Transmutation, Conjuration, or Enchantment)
Thunder-Forged weapons can offer school-specific Spell Focus +6, or universal Spell Focus +3 (Equipment).
Rune Arms have a similar but different type of Focus that isn't directly tied to a Spell School but to the Rune Arm itself. These bonuses Increase the DC of the spell on the Rune Arm at the base regardless of the charge level.