If you absolutely require version 15 (typically for compatibility with older hardware or specific existing databases), you must look to secondary markets:
Purchasing Claris FileMaker Pro 15 today is difficult because it is a that reached its official end-of-life on September 20, 2019. It is no longer sold directly by Claris or official distributors. Current Purchasing Options buy filemaker pro 15
You may find "New Old Stock" (unopened boxes) or pre-owned copies with valid serial keys on sites like eBay or Amazon . If you absolutely require version 15 (typically for