You can download a trial version of Lightroom from the Adobe site. This is a fully featured version but is limited to a 30 day trial. To convert the trial version to a licensed version you need a serial no. This you can buy online. This you then enter in the splash screen that appears each time you start the trial version As you already have the software you don't need a hard copy version. Alternatively you can go to on line stores such as Amazon and get a physical CD version, complete with serial no.
As version 6 is rumored to be released in March ( and it is only a rumor ) you may be better of waiting until the latest version is released.