As of the time of this posting Windurst is ranked 1st on pheonix. I wanted to save a few thousand gil so I came here to buy Stoneskin and it is not being sold by Kususu. I'm from sandy, maybe that has something to do with it, but the spell is not there for me.
Here's some pricing and availability figures, courtesy of Pandy's shop. ^^ These are all Tier I spells, unless otherwise specified.
Cure: 68 Gil Cure II: 650 Gil Curaga: 1,515 Gil Posiona: 200 Gil Payalyna: 360 Gil Blindna: 1,100 Gil Dia: 92 Gil Banish: 156 Gil Diaga: 1,295 Gil Banishga: 1,295 Gil Protect: 244 Gil Shell: 1,760 Gil Blink: 2,330 Gil Stoneskin: 7,806 Gil Aquaveil: 400 Gil Slow: 930 Gil
Windurst was first in the nation when these prices were taken. ^^
The country needs to be in a certain place to have certain spells available. For example on my server (Leviathan) my home country Bastok is never in first so I have to go to other cities to get spells. Windurst seems the be the best bet.