Seeking Stems and pressure sensors

[lurky]

Hello
I have a 2007 Boxster. For the last few years a local tire place has switched my snows/summer tires across one set of wheels in the spring/fall.
I got tired of the headache so I picked up a second set of wheels and I plan to switch them myself next time.
So now the summer tires are on the car and the winter tires are in the garage on the new wheels but there is a big hole where the stems should be.
I need to pick up both stems and the Porsche tire pressure sensors and I don’t know anything about stems.
-Can you use any stem with any tire or do I need specific ones?
-Where can I order the stems and the sensors?
-Can I attach the stems myself of does it require special tools?
Thanks!