Great news about the Zodiac Estucheon and Ultima too!

I don't know which Espers you already have but I'm guessing you still need Hashmal, Famfrit and maybe Chaos? Here's a quick list of Espers and their locations...

Adrammelech - Zertinan Caverns
Belias - Tomb of Raithwall
Chaos - Necrohol of Nabudis
Cuchulainn - Garamsythe Waterway
Exodus - Mosphoran Highwaste
Famfrit - The Pharos
Hashmal - The Pharos
Mateus - Stilshrine of Miriam
Shemhazai - Great Crystal
Ultima - Great Crystal
Zalera - Barheim Passage
Zeromus - Stilshrine of Miriam
Zodiark - Henne Mines

As for the sage ring, it can be bought from the stranded merchant in the Alley of Low Whispers in Old Archades. I'd pick up some Opal rings too while you are there, if you do not have any yet. Very handy for dispelling Reflect.

For Fafnir, there is a way to kinda "cheat", but you wouldn't want to do that, now would you? In case you do, I'll tell you... but I'm sure you don't want to do it!

In the Silverflow's End Southeastern part (where you face Fafnir) there is a large boulder. If you get behind it, Fafnir cannot attack you physically. Only the magic attacks (Shock, Sleep, Silence, etc.) will reach you. You can avoid them by keeping Reflectga cast on the party or by equipping Mirror Mail. From there you can attack with Scourge and Thundaga until Fafnir is finished off. A sage ring will help conserve MP. If you use this method, be sure to turn off any gambits that would cause a character to run out and attack. You want to keep distance (and the rock) between you and Fafnir. Also remember with Reflectga cast, you cannot cast onto yourself or others in the party unless you are wearing an Opal ring.

Let me know how it goes?!