There are a number of on-line vendors that carry the 3-gauge a-pillar pod.
Without showing deference to any particular vendor - I'll suggest a google search for: "wrx a-pillar pod".
Or you could just hit up some of the great vendors we have on the board:
platinum performance engineering
nino's auto accessories
Please note: I don't know if the aforementioned vendors carry the part that you're looking for - but it can't hurt to ask!