Most farmers spray their crops with something called anhydrous ammonia. It comes in huge tanks, which are pulled behind tractors and sprayed on the fields. Sometimes the tanks are parked overnight in a field. People will steal ammonia from these tanks. There are also farm supply stores like FS that actually sell and refill these tanks. Sometimes people steal from their tanks too. This stuff is EXTREMELY toxic, and siphoning it from tanks is EXTREMELY dangerous, but hey, no one ever said meth heads are smart.
As for the other ingredients, they do come from things like cold medicine. Different people buy as much as they can from different stores like Wal-Mart, CVS, etc. There has been a big crackdown on that; you have to register when you buy, and you can only buy so often in a certain amount of time. Sometimes they break into pharmacies to try to steal drugs, and sometimes they break into peoples' houses to steal their medicines as well. Like I said, it's a big problem here. I can't really describe what meth does to people....it's like no other drug on earth.