The retrofit is pretty easy. Having done it twice now (once on the 2016 and once on the 2018), my advice is to NOT remove the mirror lens like some directions suggest. I did that on my 2016 and it took forever (because I was worried I'd break it), and I wasn't sure I actually put it on back right. Just pull the door panel off, unbolt the mirror, run the wires through the mirror housing, add the signal, and put everything back on. To get the wires through, I just used a stiffer wire (I used some #18 bare wire) to pull the wires for the turn signal down through the housing.
For retrofitting, I believe Subispeed used to sell a kit, which included the signals, the wires with some pre-terminated "terminal connectors" (so you could just plug them into the existing door harness), and paint-matched covers for the mirrors. There also used to be someone on one of the WRX forums (here's the website
) that used to sell just the wiring harness (and had part numbers - I can't recall if the WRX numbers are the same as the Forester - they might be). You could do it without the "harness", but it just made things easier.