The most common way to search an auto salvage yard for used parts is to visit the yard in person and physically inspect different vehicles to locate a specific parts. Most yards feature some form of organization, such as grouping similar makes, but do not keep track of specific parts.
An auto salvage yard typically contains hundreds of different vehicles in a variety of makes and models that are in various states of completeness, as the purpose of an auto yard is to sell as much of each car as possible. As such, the process of searching for a specific part in a yard usually involves a great deal of physical searching. Before visiting the yard, know the exact model number of the part you need, and look up the different types of vehicles that use that part or a similarly compatible part. This allows you to expand your search beyond your exact model of car.
Check with the yard owner or attendant to find out how he arranges the vehicles in the yard to avoid blindly searching in the wrong area. Many vehicles in the yard are likely missing parts, so check the interior of each compatible car to see if it has the part in question. Also find out the payments methods the yard accepts before you visit as many are cash only.