The only legitimate download source is the Mac App Store using the “last compatible version” feature.
If you have ever downloaded GarageBand in the past using your Apple ID, you can download the last compatible version directly from Apple's servers: Open the on your El Capitan Mac. garageband mac os x el capitan 10116 download link
: Log into your Apple ID on a friend's newer Mac (running a recent OS), "buy" the free GarageBand app, and then log out. GarageBand on Mac OS X El Capitan 10
run the current version. "Buy" the free app there so it is linked to your account. Then, return to your El Capitan Mac and use the "Purchased" tab method described above. iLife '11 Physical/Digital Media : On El Capitan, you can also run GarageBand '11 (version 6.0.5) GarageBand 10
The progress bar appeared. It wasn't the lightning-fast gigabit speeds of modern fiber; it was a slow, churning download, the fans of his MacBook whirring to life like an old propeller plane taking off.