roadswewalk:

The root source is a site run by someone named Lee Thomson.  That all-text site that has been linked elsewhere is the raw back end; their main site is https://sites.google.com/site/tvwriting/, and this is apparently a fairly involved hobby done for the benefit of would-be screenwriters:

These scripts are here because the only way to learn how to write a
TV script is to read A LOT of other TV scripts, and there aren’t many
places you can do that.So here you can study scripts for existing shows, some of your old favourites, and many that never even made it to air.

I love running this site and sharing the scripts I obtain with other
writers and fans. However, scripts are not always easy to get hold of
(especially when you live five and a half thousand miles away from
L.A.). So I am asking; if you would like to contribute – anonymously or
otherwise – to a site that has been called “an absolute gold mine” (John
August) and “a great site” (Ellen Sandler) then please send any scripts
you may have for upcoming or classic shows to blog@leethomson.com.
I promise not to post scripts for upcoming pilots until the show has
aired, and anyone who sends me anything will of course have my undying
gratitude.

They don’t reveal the precise source of each submission; probably for source protection.  They’re also on Twitter (@ljthomson) and used to keep a blog: http://leethomson.blogspot.com/

It’s definitely an invaluable source if you’re interested in screenwriting!  

Another reference to it is here, from the Amazon Studios screenwriters’ forums.

HTH