I stumbled across these guys while reading through an Outback forum:
http://www.orange-electronic.com/en/...e_sensor_.html
They are programmable. You use their tool to read your current sensors, and then program the new sensors to match.
When the guy bought a set for his Outback in 2011 he paid $120 for the sensors, but I didn't find any information on how much he paid for the tool.
If you want to read what he had to say about them (note that his links are all dead, since Orange changed their URL):
http://www.subaruoutback.org/forums/368736-post1.html
Looks like the programmable sensor part number for the Impreza is SP101, but there is no information on their website regarding simple sensors.
Unfortunately, they don't offer web based ordering. I might give them a call to find out what's available.