Finding a legitimate DMG for 10.0.3 can be difficult as Apple typically only offers the latest version on the App Store. However, there are two primary official methods to acquire it: Purchased History:
To run GarageBand 10.0.3, your Mac must meet these minimum specifications: Operating System: OS X 10.9 Mavericks or OS X 10.10 Yosemite. GarageBand 10.0.3 dmg
This is the number one reason users search for this file. If you are rocking an older iMac, a 2009 MacBook, or a machine that is capped at OS X Yosemite (10.10) or El Capitan (10.11), you cannot download the current version of GarageBand from the App Store. The App Store will simply tell you that your OS is too old. Finding a legitimate DMG for 10
: If you have never "purchased" (even for free) GarageBand on any Apple ID, the "last compatible version" trick may not work. You may need to sign in with your Apple ID on a newer Mac, "buy" the current version of GarageBand, and then return to your older Mac to see it in your history. If you are rocking an older iMac, a