Pretty sure you'd have to buy a copy of it (even if 2nd hand). I work with it, so always had access to it. .NET however is just way below my standards of performance and minimal requirements, so I have no plans of going into it. However, you can still make plugins for RR in .NET (or any other language). But if you want to alter the current sources/plugins I'm afraid you'd have to get VB6.
Should be possible to import it, but I'm sure you'll have a hell of a time converting API code and such that isn't accepted in the same manner in .NET