Some resources that could be valuable for such an exercise:
- Daily statistics for Let's Encrypt: Let's Encrypt Stats - Let's Encrypt . 50 million active certificates, with ~500k issued per day.
- You could navigate through the millions of domains issued so far, 100 at a time: crt.sh | %
- Lists of web hosts supporting Let's Encrypt: Web Hosting who support Let's Encrypt
- Let's Encrypt has by far issued the most certificates out of any CA: https://censys.io/certificates
Some brands I found in this query:
- Techradar
- Mashable
- themeforest
- pcgamer.com
- funnyjunk
- videolan.org
- jquery.com