GarageBand has never been part of the system. It came preinstalled with a new Mac, but as part of the iLife bundle.
If you previously had GarageBand installed on one of your Macs, you will find it in the Purchases tab of the AppStore.
Is there any GarageBand version, when you open the fourth tab "Purchases" of the AppStore?
if not, you are out of luck. The currently sold version of GarageBand 10 at the AppStore requires at least MacOs X 10.11 El Capitan. Can you upgrade your Mac to this version?
You could also try to purchase the older version of GarageBand '11 as part of the iLife '11 bundle from Amazon or eBay. That will install an early version of GarageBand 6.x.x. You can update this version from Apple's downloads page to GarageBand 6.0.5. This version will run on your system.