Well I've spent over 20 years working on cars, so I can usually eyeball the part and tell you what it is. Then it is just a matter of checking supplier availability through the computer system. In the event that I do need to look up something, we have access to a lot of software systems that will help locate parts that we need. 

The way ours works here, I can plug in the make, model, year, and trim of the car. The computer pulls up a 3D rendering of the vehicle and I can tell it if I'm looking for body panels, suspension parts, etc. It then pulls up a black and white technical diagram with each and every little part numbered. You click the component you need and it will tell you what it is and the part number. Then I can go into my other software and check availability from there. Of course, these systems and software cost tens of thousands of dollars so there is no point for the DIY'er to go this route.

What part are you looking for and for what vehicle? Maybe someone here can help you locate it. The best DIY parts site that I have found is Rockauto.com. They have practically everything and are reasonable on their prices. 

Rock Auto, Parts Geek, 1 A auto Parts, Autozone, Oreilly, NAPA, Carquest, etc.