I have been using https://bunny.net/ and it has been really amazing. It might not be the biggest or fastest CDN but works really well with Discourse and the setup is minimal.
I pay less than $2/month as the pricing is a minimum of $1/month and based in traffic.
I use a cdn.<domain.com> URL and they manage the SSL via Let’s Encrypt once you prove you control the domain.
Referral link: bunny.net - The Content Delivery platform that truly Hops!