Uptime Kuma is a good open source monitoring tool if you'd rather self host. I added NIP-04 DMs to it as a notification provider too.
If you run a node (that accepts inbound connections) you can set up a monitor to check port 8333 for free at https://uptimerobot.com/

Uptime Kuma is a good open source monitoring tool if you'd rather self host. I added NIP-04 DMs to it as a notification provider too.