I'm trying to purchase Keynote for Mac Os 10.7 How can I buy an older version of Keynote that's compatible with 10.7?
You can purchase the iWork suite (which includes Keynote) from Amazon or a similar retailer. Searching the internet should help you, but I found iWork '09 for $19.99. Note that iWork '09 uses universal binaries so it will work on both PowerPC and Intel Macs.

You may want to check which versions are compatible with OS X 10.7. Wikipedia has a good table with OS X / iWork version compatibility.

You need either iWork '09 or iWork 9.x. If you are on OS X 10.7.4, you can run any version of iWork up to and including iWork 9.3. 

iWork 2013 is only compatible with 10.9, so stick with versions prior to that one on Amazon or another retail site and you should be good. If the version you want is not on Amazon, check eBay or another website.