I've not found a free one that works well. http://rss.app works reasonably well (it's always a gamble w/ how the service parses/scrapes the pages...). I always have better luck when the page supports RSS out of the box... but this can be hit/miss.
I am considering updating the work flow to skip RSS all together. the general idea is to:
scrape the OTA page and keep track of the most recent build number / the total number of "seen" build numbers.
if the number of build numbers parsed is higher than the seen number, create an entry in my todo service.
it's mostly written... i just need to get some spare time to turn it into a lambda or similar so it's easier to host.