Is it still possible to purchase a license for Mac OS X 10.6 Snow Leopard Server from Apple?
Snow Leopard Server was never made available via the App Store, it was only ever sold in the form of a separate OS install (or bundled with a server computer). While you cannot buy SL Server from store.apple.com, I believe you can buy it over the phone (1-800-MY-APPLE) (at least, you could last year -- I did) for $19.99 (maybe plus shipping, I don't remember). Note that you're purchasing the physical install DVD, not just a license code.