https://zerossl.com/ seems to be a popular one. Just click “Online tools” and “Start”, fill in the form and follow the instructions. Then consult the documentation @JuergenAuer linked above for installing the resulting certificate on your server. Remember, Let’s Encrypt certificates are valid for 90 days, so you’ll need to repeat this manual process to renew your certificate at least that frequently (best to renew early to leave some time to recover in case of problems).
At some point you might consider switching to a hosting provider that supports automated renewals.