If you previously owned or downloaded GarageBand '11 (v6.0.5) using your Apple ID, Apple still lets you redownload it. Open the on your Mac. Click on your Profile or the Purchased tab. Scroll through your history to find GarageBand .
If you already own it via a previous purchase or if it came with your Mac, it is free to download from your App Store Purchases tab. If you never owned it, you cannot buy it from Apple anymore. Garageband 6.0.5 Download
If you previously "purchased" or owned iLife '11, you can often find it in the "Purchased" section of the Mac App Store. Clicking download there may prompt you to install the last compatible version for your specific hardware. If you previously owned or downloaded GarageBand '11 (v6