Snow Leopard Server

At one point they were selling the SL Server version by phone only through the business division. Meaning you couldn't buy it directly through the Apple website. I just told them I was buying it for the company I work for and I didn't have to have an actual business account. I don't know if they've discontinued doing it this way by now, but my guess is that they have stopped selling it all together. You could try and call the business phone number and see. It's at the bottom of this page. https://www.apple.com/retail/business/

I went ahead and bought the disk several years ago when they were offering it that way, since it was only $20 and knew I might need it in the future (mainly to run FreeHand MX in a VM, since we have over two decades of graphics files in this format). Even if you found an installer, unlike other versions of the OS, the SL Server version requires a serial number to activate. So it may even be dubious to buy a used disk online from someplace like Amazon or eBay.

The only Snow Leopard software available from Apple is the Snow Leopard Installer DVD you can purchase through the App Store. Snow Leopard is obsolete.