The best way to get Mirror Mail is not to buy it, but to chain Mirror Knights in the Feywood. From the sounds of it, the EXP will still be worth your time and the loot you get can be sold for a hefty sum of gil. If you want to just get on with it, I think the store in Archades/Phon Coast sells Mirror Mail.

There isn't really a "best area" to my recollection, you're just moving to the opposite side of the screen of him while making sure you can still hit with Magick. Fafnir is incredibly slow, so you should never find it difficult to keep out of his way, especially if no one is actually bothering to attack properly.