Parking
The first piece of evidence is tough to get. It's in the parking lot on a stack of boxes. There's really no way to get there without a disguise.
Reception
Similar to the last one, this is on a stack of boxes near some vehicles. it is almost directly in the middle of the area. It won't be destroyed if you blow up the gas station as part of the mission but trying to get to it will be very difficult no matter what you do. You'll likely need a disguise.
Cornfield
This is significantly easier. Find the road that runs through the cornfield and then find the two trailers at the intersection. In between the two trailers, in the middle of the road, will be this evidence on a stack of boxes. You should be able to get to this fairly easily. You may have to remove a couple of guards but no more.
