Scavenging is the act of collecting usable items that still have life to them or can be re-purposed. Examples of scavenging that we see every day is recycling and reuse it centers. Native Americans use to use the remains of animals they ate to make tools. Certain animals such as vultures and raccoon are natural scavengers. So it is a very primal act that eventually we may need to succumb to in order to survive and thrive.
If you are working alone it is always going to be best to search and scavenge abandon areas with low levels of activity, however if you are in a survival group more elaborate plans can be made to seize supplies from rebel, military or other government facilities.
For mechanical and automotive repair parts junk yards and auto body shops are a good place to find parts and equipment. Fuel can be siphoned from abandoned vehicles, gas station reserves, transport vehicles and treatment facilities. Energy is often overlooked since today we take power for granted, so gas generators and solar panels are high priority in maintaining power.
Ammo scavenging can be a difficult operation as it includes targets such as law enforcement and military installations that may pose a significant threat. Alternatively anyone with a bullet press and casting setup can effectively manufacture their own ammunition with scavenged scrap lead from junk yards (wheel weights on older cars) and medical facilities (radiation shields made of lead).
How to scavenge for Food is probably the easiest of all, hitting abandoned stores and bix box locations (make sure to check the storerooms). Abandoned houses, dumpsters, farms, etc.
For medical supplies hit doctors offices, hospitals, veterinary offices, pet stores, drug stores and other retail locations. Distribution centers, restaurants and warehouses are all potential targets for scavenging as well.
For survival supplies check big box stores, home improvement locations, sporting good retailers as well as any general structures you may come across.
An overlooked aspect of scavenging is structure, any abandoned structure you come upon could be used as a base, so keep your eyes open for suitable fortresses with adequate furnishings and design. Locations suitable to hold out against an attack for an extended period of time, complete with external and internal defense are a good place to settle with your survival group.
Scavenging in areas of urban decay in order to survive and rebuild may very well be necessary in our uncertain future with that of world wars, civil conflict and disaster. Establishing a SHTF plan can be crucial in being one step ahead of trouble.