The Tribunal expansion adds Elbert Nermarc, who sells leveled items, including the ring, along with other items, that only become available for Tribunal Temple members once they are past rank 8 (Diviner). But the few selected leveled items present in his inventory are randomly generated, therefor making it difficult to obtain.
It can be can be acquired on the PC by using the console:
Player -> AddItem "st. felm's fire" 1
- This ring does not appear in Morrowind.