They made the One Ring for the Lord of the Rings movies. They make replicas for fans as well as inscribe custom tengwar messages onto jewelry!
I helped them improve their English auto-transliterator, though it should be noted that a computer program can’t sense context, and won’t be perfect. So, when you select their “Elvish Inscription Checking Service” option, I will be personally transliterating the inscription. You can also send them one of my translation+transliterations, either from the main website or one that you had custom made, and they will inscribe it for you!
If you are looking to getting a set of rings inscribed with tengwar, I can personally recommend you get it done with Jens Hansen!