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.