Can I still buy the "Diplomacy" expansion?

Hi.

I guess this is directed to the Stardoc people. I haven't rly played Sins since Entrenchment but I'd like to start playing again this time with less of a traditional rts focus. Basically I'd like to explore the more diplomatic aspects of game play and so it probably makes sense that I get the Diplomacy expansion (micro or whatever its was called). I don't seem to be able to find it for purchase on steam or via this site here.  

Can I not still get it? 

All I have found is trinity and I don't need the whole thing, just Diplomacy. 

I looked into getting Rebellion but money is tight and tbh I just don't feel there is enough in it to warrant paying 30 or 40 dollars for it. Maybe if I was planning to to do multi-player (as I assume thats what most people are playing) but I don't see myself doing that. 

Thanks in advance!