It is still available at CBD and likely elsewhere.
The NASB SB (and Zondervan's KJV Study Bible) is based on the 1995 10th Anniversary revision of the NIV SB. I've heard that it doesn't quite have all of the NIV SB resources.
I think that Zondervan NIV/NASB/KJV Study Bibles are ok as a "training wheels" Study Bible, but they punt on too many controversial issues and has always seemed to me to be aimed at a lowest common denominator evangelicalism. No doubt, that is one reason why the NIV Study Bible became so popular. Back in 1985 when it was first published, about the only alternatives were Scofield, Ryrie and some others that have long been forgotten by most people.
There weren't as many choices in the late 90s when it was published. The Reformation Study Bible was only available in the NKJV, as was the MacArthur. About the only choices in the NASB were things like the Ryrie, which would obviously be unacceptable to Reformed people. I do remember the book introductions in the NIV Study Bible being helpful, especially for someone like me who was just beginning to study the Bible and had had to read the New Oxford Annotated Bible in college with its promotion of the documentary hypothesis, denial of traditional authorship of the NT, etc.
There is supposed to be a new NASB Study Bible coming out following the release of the upcoming NASB update. I don't know whether or not it will be a new SB or an adaptation of the NIV SB or some other existing SB.