Available directly from the vendor.
It's the cheapest smart thermostat I've seen, and definitely benefits from not needing a power wire.
I use a similar one that cost $80 and am very happy.
On sale with 25% off with code HOT25, $35 plus $8 shipping
EDIT: If deal is OOS, try Ebay (thanks vttimc):
Buy It Now = $45.60 shipped
(They also have open box for a few bucks cheaper)
I originally found it at Amazon for $54, with a switch.
Apparently, the switch is useless without an Iris hub, but the thermostat works directly with ST. (There might be some ability to program a custom handler, but this looks to be for a different model.)