In general, retaining records of source documents such as W-2 should be your clients' responsibility and not be covered by the tax preparation fees (even though some of those documents would be kept on your file for professional practice purposes).
To access the relevant documents easily, you need an efficient filing system and the method you use would differ depending on whether you utilize a manual or electronic system. It's all about having a system for organizing folders and categorizing documents, a consistent convention for naming files, and control for versioning.
Still an AllStar
DMS, SmartVault, eFileCabinet.... There are a plethora of software tools to help set this up. You can also create folders in window for each year, then under that each client then for each client, Return, Documents, Communications, etc.
The idea of having the ability to attach a pdf of a client's W-2s, 1099-Rs, and any other forms we are required to keep, to the 1040 file, has been my wish for many years. I imagine it would be necessary to attach the pdf after the return had been filed electronically.
I've always (at least for several years) scanned all clients documents into ProPDF folder in appropate year, and named it the clients SS number. I also print all returns to printer and PDF, so I will have a copy if ever needed. Have auto backup to external HD