You can buy Little Favour on iTunes. Taking pre-orders of 5-November-2013.
VODzilla has a short piece about Little Favour, which includes the official trailer, in case you missed it.
ScreenRant is a bit more spoilery. As is WETA, but hey, with news like this!
Benedict Cumberbatch’s Short Film ‘Little Favour’ to Get Worldwide iTunes release
And this is what I was looking for -- how high up Little Favour is in the iTunes. From PREORDERS. Holy mackerel! Here's the press release as of 24-Oct-13.
Number frickin' one!
BTW, some of the info on these sites will be duplicated. I've read them already.