Parts list for buying parts?

If you get Apple certificate training you would gain access to Apples technicians guides for the each of the iPhones which have proper parts listings. 'You would also have access to Apple spare parts and technicians online genius help service (for those real tricky problems). While the Apple supplied parts are expensive you know your getting the correct working part.

If you plan to do this as an occupation then I would strongly recommend signing up (there are some costs) but in the long haul it will pay for its self.

If you just want to fix one or two phones then you have sites like IFIXIT here which offers extensive teardown and repair help guides. While they don't offer parts listings, other sites can give you partial lists of the replaceable (serviceable) parts. Getting parts from 3rd party suppliers can be tricky as you often don't know if the part was salvaged or new. In some cases they are rejected seconds which may or may not work or could have slight blemishes. Lastly, you might get someone who takes your money and sorta forgets you wanted something in return. But many do come though with good working parts as well.

One last point: leaked copies of Apples service docs are also findable on the web. Try doing a Google search for: Apple Technician Guide iPhone4 or what ever iPhone or iPad you are looking for. There you will find a exploded breakdown of the parts and discreet pictures of each of the screws and many of the parts.