The Stardock Central has a "Lookup serial" function in the registration part. When you are registered at Stardock, this is an easy way to get your serial back.
When you are not registered, just buy the game again. In Europe I saw it for 10,- € in stock or for 30,- € including AP at ebay. Stardock itself offers the Ultima Edition including AP for 30,-$ here Link .