ok. well do you have the disks that came with your mac? you can install garageband from there.
Instructions:
1. Insert Mac OS X Install Disk 1
2. A window will appear. (if it doesn't show up, navigate to the disk in finder). Choose "Install Bundled Software Only". An installer will open. On the screen that gives the button saying install, DON'T ClICK THAT! You want to choose "Customize" (As shown in screenshot). Place a check mark by Garage Band and any other items you want to install.
3. Now choose "Install". Thats it.