In my region, you need to use a dashboard by Countee to get informed about vacant vaccination slots. I have built counteeexporter on a whim because it is a waste of time to check the official dashboard several times a day only to find out that there are no free slots. It leverages the Countee API to fetch the information and exposes the results in the Prometheus exposition format. That means you can use counteexporter as a scrape target for Prometheus and create automated alerts for vacant slots in a vaccination centre of your choice.
You don't need to set up a Prometheus instance yourself to use counteeexporter. My recommendation is to get a free Grafana cloud account. You can use the Grafana agent to send the collected data to your managed Prometheus instance. Check the deploy branch to get an idea of how you could set this up.
The screenshot below shows, that several hundreds of vacant slots just appear out of the blue after a long period with no availability. You need to be really fast in metropolitan areas because they are booked within minutes. Here is a snapshot of the timeframe displayed in the picture.
Disclaimer: I did not find official documentation for the Countee API so the exporter probably only works with the endpoint I was interested in.