A few days ago, on IRC, I found out that Harica is providing Domain validation for onion sites for around 30 per year. For the common name, provide the same onion address. After payment, you should be able to download the certificate with the full chain (the file ending with.p7b) After the click on the website, it will ask you to download a file and put it in your web root inside of the well-known/pki-validation/ directory.”]
Source: https://kushaldas.in/posts/get-a-tls-certificate-for-your-onion-service.html