There are a few sites offering similar services. I used to use
www.downforeveryoneorjustme.com when TalkTalk was flaky. It was usually their DNS servers, which are generally the fastest for me, but they tended to drop out at odd times.
I also like this little tool:
https://www.grc.com/dns/benchmark.htm and have used it to setup alternatives for DNS. I built my own custom list and have saved a few IP addresses. They are now my first port of call when all sites are unreachable.