As has been noted in other threads, Tesla sells snow chains for the Model S HERE. They are $89 for a pair, plus $32 shipping, plus tax.
The picture clearly shows that they are SCC Z-chains, so I went to SCC’s site and looked up the size. They are Z-563. Then I searched around for the cheapest price; I found $85 including shipping (but not including tax) at Amazon, and ordered a set.
Here are some pictures. The chains include gloves and instructions. Installing chains is never easy, but this was as easy as (or at least no harder than) anything I’ve installed before—especially after I thought to raise the suspension. They seem to fit well. First, the bag they came in, including the "Z-563" tag:
They come with instructions and gloves:
Here is one spread out on the driveway. The red side goes on the inside of the tire. The other side is split in the middle, so you can put the chains on the ground, sweep them around the tire, and then connect and tighten them top and bottom in the front. (You do still have to connect the back side at the top, obviously; but no tightening or anything back there).
And here they are mounted:
And again, from the back of the car so you can see the cables over the tire:
They are small so you can stuff them about anywhere; but they happen to fit nicely in the little cubby in the left rear that typically holds the charging cords:
Obviously I put them on the rear wheels, because they are the drive wheels. But I wonder - should I get a second pair for the fronts in case conditions were really bad and I was worried about losing steering? After looking at the front tires, I think not. I can physically get them on the front wheels, but part of the suspension wraps around the tire and I don’t see any way the chains wouldn’t rub.