: If you never "purchased" it, log into your Apple ID on a newer Mac (running a recent macOS), download GarageBand there to link it to your account, then return to your El Capitan Mac to find it in your Purchase History. Compatible GarageBand Versions for 10.11.6
Searching for “GarageBand El Capitan 10.11.6 download extra” will lead to sketchy sites like: garageband mac os x el capitan 10116 download extra verified
a used copy of the iLife '11 installer disk from sites like eBay or Amazon . : If you never "purchased" it, log into