It's a 3.0-era free download from the WotC site
Though I originally thought it might be from an article or web enhancement for a 3rd-edition D&D book (thanks to its rules wording and use of the 3.0 softcover page dress), it did not show up in any of the existing article archives on the Wizards of the Coast website, nor in any of the 3.0-era web enhancements.
After looking through various posts that reference it, and running the link through backlink checker sites, the conclusion I came to is that the source might simply not exist anymore.
Using the Wayback Machine to looking at the D&D part of the Wizards of the Coast website from 2001, I found a page of free downloads for players and DMs. The "New Magic Items" pdf is listed under the magic items section of the page, as "A Magical Assortment":
Name: A Magical Assortment
Description: Wizards of the Coast presents new magic items for 3rd edition D&D: amulet of spirit storing, catspaws, falcon cloak, mirror of scrying, mantle of rage, ring of nine lives, and reflex weapon.
Author: Stephen Kenson (email@example.com)
Associated Link: http://members.aol.com/talonmail
Setting: General D&D
Category: Magic items
As a final note, most of the downloads on the linked page are unofficial. Per the top of the page:
Unless otherwise noted, these downloads are all created by fans like yourself. They might break rules, cause your campaign to become unbalanced, or create questions for which there are no immediate answers. Use at your own risk... and remember that your Dungeon Master has the final say as to what he allows to enter his domain. If you have questions about unofficial downloads, please send an e-mail to the creator.
A Magical Assortment calls itself out in its description as being presented by Wizards of the Coast, and is written by Stephen Kenson, someone who had written for WotC at the time and is now one of the authors for D&D 5e edition books. It also remains in the WotC site's files, unlike the rest of the page. As such, I personally feel that this, unlike most of the links on that page, was meant as an official 3.0 article.