Congrats on your D90. They are great vehicles as you will soon find out. The top you are looking for is called a Tickford top. It is the one that fits the 94 D90s without any modifications as they utilize snaps. Later D90s use a soft top that have the rail system and it is made by Best Top (same manufacturers for the Jeeps and other vehicles).
You can also fit the Best Top soft top on a 94 if you convert your snaps to the rail system. But the Best Top (as fas as I know) doesn't have the "Alpine Windows".
Both tops are available still either from the Dealer (expensive) or places like the one Yousef posted above or Rovers North. Used ones are sometimes on eBay from 800-1200 depending on condition.
Another good option that people seem to like is the Badger Coach Works top (http://www.badgercoachworks.com/page5.html
). Expensive but great quality. They can run over $2K.
Good luck and enjoy your new toy!!!