http://www.subispeed.com/subaru-jdm-dual-console-arm-rest-extension-2015-wrx-2015-sti New it goes for $174.95 + shipping from Subispeed. Which is crazy for a little plastic cubby, but it makes a huge difference in comfort if you're tall like me, and gives you an extra storage compartment. I never regretted spending the money. Unlike the USDM armrest extension, this reuses the cover from your existing armrest, so it you have a leather interior, it'll still match. In like-new condition with all fasteners and trim. Asking $75 OBO.
Well, that was a fail. Twice, actually. Forgot I didn't buy the armrest from Subispeed, but direct from Japan, and it was a lot cheaper, so, update price to $75. And for pics of it installed go look in the "what did you do to your subaru today" thread because I'm too lazy to figure out why the image links worked when I put them in and now don't.