10.6 Server can be found at various online retailers including Amazon at around $200-$300. That should work well on your Mac Pro - you can use Server Admin and Workgroup Manager to set up and manage users and sharepoints.
Apple's documentation is still online in various locations - I also love the Wazmac guides available here.
I believe 10.7 Server was only ever available as a download or pre-install (I don't believe there was a USB key version of it as there was for the client OS X), so you might be out of luck there.
If you are supporting fewer than 10 users, you may not even need the Server software, you may be able to do that with File Sharing. You should seriously consider gigabit Ethernet for the switch connecting the computers (not needed for the Internet). Don't use wireless for serious File Sharing, it is not speedy enough.
As for availability, I expect 10.7 Server is sold the same way 10.7 is sold -- CALL the Apple Online Store and i expect they will sell you a download code and tell you where to apply that code to download it. MAKE A DVD before you let it install. Server Admin Tools is a separate (unrestricted) download, and it will only administer the same version of Server.
Thanks Matt Clifton: I will go with 10.6 and try Amazon.
Grant Bennet-Alder & infinite vortex: Unfortunately, File Sharing will not be enough. I work within a LAN (mainly Windows) which has thousands of users. Access to the server has to be regulated. Speed is not an issue coz we're connected to a 100Mbps network. Another issue is I'm based in the Middle East and sadly, there is no Apple Store in the region.
Occationally, users may need to access files when they are in the States or in Europe. Any upgrade considerations ragarding this?
Thanks again for your advise.
Regarding App Store access, you may be able to obtain an online purchase if you have a credit card based in a country which does have the App Store. (Doesn't require having a local Apple Store.)
For remote access, the most secure option is a VPN connection. It creates encrypted, password-protected access from a remote client to your router. Find a router/gateway with a VPN server. (I've been using the Cisco/Linksys RV042 model at several clients with success, but there are many alternatives with different feature sets.) If you have fixed offices at other locations, they can install the same gateway, and then you would create a gateway-to-gateway connection. If they are roaming, they can use VPN client software such as the free IPSecuritas (or VPNTracker, not free) for Macs.
The insecure option is to punch a hole through your firewall which permits file-sharing and directs it to your server (port 548 for Apple File Sharing). You then give your clients your public IP address and they connect directly to your server that way. Again, not secure, so use with caution.
Both options work best when you have a static IP address - if it's dynamic, look into dynamic DNS services which give you a permanent address such as ouroffice.dyndns.org. Dyndns.org has some reasonably priced services.