I got mine through Script City, which is located in California. Although I own the first, third, and fourth drafts of BTTF, "Paradox" (the draft of the sequels when they were one film), an early draft of BTTF2, and an early draft of BTTF3, not all of these may be aquired through SC. (The one of BTTF3, in particular, which I acquired online through a place called "The Script Store," but whose web page URL doesn't seem to work anymore.) I also bought these screenplays back in the mid-1990's, and I'm not sure how many of these drafts are still available today. Check the website and call or e-mail them if you have any questions. Even back when I ordered mine, some of the drafts they had were not listed in the catalog at the time.
However, one may download, free of charge, the fourth draft of BTTF and the "Paradox" script, courtesy of BTTF.COM. They are in the Adobe Acrobat format, which must be used in order to view it, but this can also be gotten for free. I also novelized the first draft of BTTF back in the day and have it posted on my webpage. It is a novelization -- not a screenplay. Okay? (Some people have posted this erroneously in other spaces on the 'net, much to my being ticked off.)
Once you go through the work of acquiring them, I think you'll be happy. They are very very interesting reads, with extra information on the characters, a lot of scenes that never made the cut, and -- in the case of the very early drafts for the original Back to the Future -- almost a completely different film altogether!
If you can't wait a few weeks to get your hands on the screenplays, or if they are indeed no longer available, I have summaries written out on all the screenplays I've read, as well as the official novelzations, at The Future That Never Was. But beware -- if you want to be surprised while reading the screenplays the first time, don't go here! Spoilers lurk within and it'll ruin the surprise.