You have to find them in this order. The next one in sequence won't appear until you've found the previous one.

1) In the barrel by the gas station


2) In the window of the store


3) Behind the hospital on top of the dumpster


4) Stuck to the high voltage pylon behind the stores


5) In the lone house on top of the cliff (in the room with the piano)


6) The cross on the grave in the hills up behind the gas station (follow the valley behind the oil tanks)


7) This one is a key. It's in the house with the broken fence and barrels out front.


You get the actual seventh note once you open the door of the hospital, then turn around and read the note next to the door


There is more after this but it's pretty linear.