> ZeroSSL.com
I kinda wish people would stop recommending them. This might have changed, but last time I tried ZeroSSL (~a year ago) it was not RFC 8555 compliant (specifically section 7.3.1), and you were basically supposed to use their own proprietary API to deal with the issue. So you can't always switch transparently.
If you need an alternative use Buypass. Also free, and they're actually RFC 8555 compliant.