Tire Pressure Monitor Question

Just took delivery of our '06 Barcelona Red Prius, #6, a couple of weekends ago and as I have been reading the owner's manual , I realized that I don't know and the manual doesn't say where the tire pressure monitor is. Is it a small transmitter inside the tire or in the valve stem, and does it have to be replaced each time a tire is replaced or fixed? Also, does it have to be done by Toyota Service?
I lurked here and other websites gleaning what info I could before buying. We rented a Prius before deciding to buy and are very happy with it .

 

Thanks Galaxee for the short answer and the link to the thread. Will tire dealers need to be told about the valve stems or will they know what they are when I get new tires and/or a tire fixed? We are still happily learning lots about our Prius.