In posts people keep talking about alternate scripts for Godfather II, especially involving Fredo and the assassination attempt. Where can I find such materials?

I have only one script, second version as far as I know, it's here.
BTW, great signature!

There are plenty of places on the net that sell movie scripts. Here's a good one to check out:

MOVIE SCRIPTS FOR SALE

The third draft of Part I (dated March 29, 1971) and the second draft of Part II (dated Sept. 24, 1973) are the ones most available.

If you ever walk through Times Square you'll find a few street vendors selling these scripts (for a few dollars less than the internet sellers).

I think one of the best things about the alternate GFII script is how Tom communicates his disconnection to Michael. We see Tom having an affair with Sandra (Sonny's widow) and how Tom complains to Sandra that Michael no longer needs Tom. There's a poignant scene in which all of the info Tom gave to Michael about Sen. Geary was wrong. The ensuing frustration Tom shows is discussed.

I also liked how they delved into how Anthony was not very happy, even as a child, about his father.

I would encourage all to read it who like GF & GFII.