If you just opened a box and a piece is missing or broken, LEGO will usually send it to you for free.
When LEGO no longer stocks a specific part—especially for older or licensed sets—these community-driven sites are your best bet. How to get pieces that are missing from a new set - LEGO
How to get pieces that are missing from a new set. Missing something from your new LEGO® set? We can send you the pieces you need!
2. Third-Party Marketplaces (Best for Rare or Retired Parts)