No official cheat codes for Dragon Quest IX. Action Replay / Cheat devices can corrupt save data permanently (checksum mismatch) and risk online ban (DQVC / Wi-Fi).
Farm Seeds of Strength, Agility, Defence, Life, Magic from High-Level Grottos (Lv 80+) / Legacy Boss drops / Alchemy. Max stats = God Mode.
No cheats needed. The grind is the game. 200+ hrs for 100%.