This is exactly why I built NamePromo [1], which tracks product stock changes and sends you alerts across many channels (email, sms, mobile push (iOS/Android), our FB & Twitter profiles, and Discord).
We also take requests for any store listing URL and even if we don't support it right now such as Nike for example we will prioritize it and get it added to our queue to support tracking it.
The interesting thing about your point that Amazon and other merchants that have a "notify me" button is that they don't really work for users. However most of their methods are to process new inventory, their notification process is disconnected somehow and delayed for some reason. From the community I've been working to build as well, they all say the platform is much faster than the stores' versions.