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This is a good question. In all of the Pathfinder Chronicles and other books that I have read, the NPC levels usually stop at around 10-15th level. In Mythic Realms it does talk about several NPCs who are 20th level casters, although these people are all equivalent to god level and therefore not likely to spend their time writing up scrolls to sale for minimal (for their power level) prices. The most common source of high level casters is at the Mordant Spire, where "a typical Prime Censor is an 18th level elven wizard" (Mythic Realms 12).
On the whole, 9th level caster's aren't really readily available anywhere. In the Pathfinder Core Rulebook it says that "Even a metropolis isn't guaranteed to have a local spellcaster able to cast 9th level spells." (Pathfinder Core Rulebook 163)