Hand of Vecna. <Just kidding>
Short of a high level spell to regenerate the arm, your choices are (from easiest / least expensive to hardest to procure / most expensive)
A hook
A stump cover with accessories (plug ins of different tools)
A mechanical (non magical) arm, most likely clockwork
A graft (a number of source books have grafts - it wouldn't look like an arm but a tentacle or claw have been described)
A magical surrogate arm - I think Complete Warrior or Fist & Fury had a magical silver arm. Another poster has mentioned other sources.
Hand of Vecna (ok, I wasn't kidding......)