This a bit old, but I'm betting it still applies...
I had this problem also. We have a 2008 Xserve (Xserve2,1) that doesn't meet specs for 10.8 Mountain Lion, so we had to get 10.7 Lion. DO NOT CALL SALES, not even the Apple Business Store. They do not know how to order 10.7. Once you find the right person to talk to in support it is very easy. The only problem seems to be finding someone who knows how to place the order for you.
Also, be sure you get BOTH 10.7 Lion and 10.7 Lion Server.app, about $20 each. They'll send you an email in a couple of days with the download link and redemption code. You have to do this on a system with at least 10.6.6, otherwise you can't get to the Mac App Store to actually do the download.
Re: Can i still purchase Lion Server 10.7?