Before his death a few years ago, Victor Bove was a good source for tagging varieties. Another I have used is eBay
seller tbrstamps. He specializes in modern definitives, but then most issues that have multiple tagging varieties (except "tagging omitted" errors) are definitives. That said, I have seen him offering commemorative varieties on occasion. In addition, he frequently offers complete "tagging sets" for an issue. In his listings for these, he usually includes a detailed description of the different varieties. When I started collecting tagging varieties, I often would find his eBay
listings a useful adjunct to the Scott catalogue listings.
I generally try to keep abreast of tagging varieties by checking the Additions, Deletions & Number Changes
page in the Introduction to the Scott US Specialized Catalogue. I only purchase a new Scott every 4 or 5 years, so I photocopy this page from the public library's copy in other years. Newly discovered tagging varieties invariably show up on this page.