After purchasing the retail versions of the films:
Download the .iso files from wherever you can find them. You'll need to burn them to Verbatim DVD+R DL discs. There's a helpful video on youtube that will walk you through the process. Just search "How to get Original Unaltered Star Wars Trilogy on Blu-ray." Ignore the blu-ray in the title, this video will explain how to burn to DVD+R DL discs.