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.

I can confirm that you can still buy Snow Leopard Server with an unlimited client license over the phone. I just did so today (2014-12-04) in Australia for a total cost including delivery of AUD24.95, about USD21. The sales person that I spoke was knowledgable and knew exactly what is was without me having to quote the part number. I think that he may have had a scripted question and asked me whether I was using it in a virtual environment such as "Parallels" or on physical hardware.

EDIT: I just got off the phone today (2015-01-27) with Apple, and Apple Tech Support (USA), and they are no longer shipping Snow Leopard Server. Suggested I look for a copy on eBay.