For the most part, the email "alerts" are useless, because, by the time you receive them the product, if really limited, is already sold out. Unlike Iwan Ries, Smoking Pipes and other on line stores won't hold a wish list blend for you to pick up when it drops. Iwan Ries will take your wish list order, put you in their queue, and when your turn comes, will hold your order and give you three days to claim and pay for it. The rest won't do jack shit.
So how do people know of a tobacco drop days in advance? Like the rest of everything, it's social connections. They've made friends with people working at the stores or the distributors, and get advanced warning. If you know any of these people they'll share that info with you, as long as you don't spill it. And if you do spill it, you're out.
If you've been a good customer at a B&M, the owner might reserve something you're wanting to have, when it comes in. Again, it's social connections. I used to send the owner of one of the B&M's I shopped at, gifts of some tobaccos that he couldn't get in the US, as well as some out of production blends I knew he liked. In return he set aside for me some hard to get blends when they showed up and gave me a call. I got what I wanted while other people got zip.
The threads here are great for instant notification once something has dropped. But often, by the time it gets posted here, I've already finished my buying.
Connections are where it's at for advanced notice. Start building them.