Finding scripts of films that have yet to be released is going to be difficult online. Hell, even being in development within the industry there are certain scripts that are impossible to get (Marvel, Anything JJ, more and more of the tentpoles). High profile projects are typically read on an iPad that has been locked where you can only read and not take any screengrabs. There's other programs out there that will have the script delete automatically after a set timeframe and/or not let you go back once you've read onto the next page. Lastly, some high profile projects go under code names (e.g. The Han Solo project is listed as "Red Cup"... Get it?)
But it's not impossible. The biggest leaks come from when the script is sent to casting. Though the bigger scripts only use sides and don't send out the whole script. So if it's before casting, the script has more than likely only been seen by a handful of people (agents, their assistants, and those on the project at the ProdCo and Studio). High profile releases like Hateful Eight happen rarely, but not usually on that big of a scale.
So how do you get these pre-release scripts? Connections to people who work at ProdCos/Studios, casting and agencies. There's a small chance you can find scripts on DoneDealPro, but since the Deadpool lawsuit, I doubt many would share/trade on there anything that is higher profile. Sites like PDFScreenplays went dark after that lawsuit as to protect themselves. But I know there are still some private script trading groups out there, I just don't know who or how to find them.
You can also find almost any script that has made the blacklist in years past, as there are always MediaFire or Dropbox accounts that hold them all. You just have to be savvy at searching.
If it's a film that's been released, you can search the script title +PDF (as it is always best to read in PDF format and not html if you are reading to study the format).Ā You can also check out sites like Drew's Scipt-o-rama (does that even still exist?), Internet Movie Script Database, and there are even sites that will sell you a copy (ScriptFly, etc), but I don't really support the ones that are making money off of a script that they don't own the rights to.
If you have any more questions, shoot 'em here, or if it's something longer, you can always email me zac at zacsanford dddoottt com