12 degrees ... C or F? Big difference.
What about getting a basic long-sleeved jersey? When the temperature starts getting a little bit cooler, I will usually wear a polypro base layer with a long-sleeved jersey over the top. Torpedo 7 has long-sleeved jerseys with a windproof layer in front which helps with the warmth factor. Then I wear a vest or jacket over that depending on the sun, wind, etc.
Even cooler and I'll do polypro, then merino wool, then the long-sleeved jersey, and then jacket. Cold ... and I'll add a fleece layer in between the long-sleeved jersey and jacket. It's all about layers.
I actually prefer polypro to merino wool as a base layer, and you should be able to pick up polypro at Sierra Trading Post or Anaconda/MEC/Decathlon/REI. You might even be able to get it at Walmart or Canadian Tire or similar places ... maybe ... it might be worth a look.