This is pretty simple for the 11-up WRX. You need the signal units, and the mirror covers (need to get them painted though). You can get those parts new from Subaru or you can save a few bucks and pick them up used.
You do have to figure out what you need to do for wiring, from the mirror plug to the signal unit. Monkiboy and I have both brought in a number of the JDM mirrors for members here and elsewhere, commonly they have the signals, the nice thing about buying them from a used set of mirrors is you can get the wiring with the signal units to make it just like stock. I am working on sourcing the connector for that so I can make new harnesses for that. I might have what you need in stock PM me if Monki doesn't have stuff for you.