How can I buy an older version of Keynote that's compatible with 10.8? My work computer can't be upgraded to 10.9 yet, so I'm stuck on 10.8 for now. I've never used Keynote, but one of my coworkers wants to use it for a presentation we have to give. I just tried to buy it, but I'm getting an error that the current version (6.0) isn't compatible with 10.8. How can I buy an older version of Keynote that's 10.8-compatible?
I presume that you're trying to purchase iWork from the App Store.  

You can purchase a physical copy of iWork '09 and install it.  

If your Mac doesn't have a disc drive, you can use Remote Disk to do so.  You'll need access to another computer (either Mac or Windows) that has a disc drive.  Apple has complete directions for using Remote Disk here.