There is no Snow Leopard Server 10.6 Server download, nor is there a download for the 10.6 client edition.
First, ensure that your system is supported by Snow Leopard Server. The Mac OS X 10.6 Server Admin: System Requirements for Installing Mac OS X Server (PH8190) web page includes the specifications for 10.6 Server.
(For readers folks that can land here with a search looking for OS X client 10.6 upgrade details, see the Mac OS X v10.6 Snow Leopard - Technical Specifications (SP575) web page.)
For details of your Mac and its configuration: > About This Mac > More Info
Then call Apple, and order the DVD.
In the US, the contact number is 800-MY-APPLE.
The other obvious alternative to the software upgrade - and which will bring you a newer version of OS X Server - can be a used or new Mac server box. The recent Mac Mini Server is a gonzo fast box.