|David Montgomery RPG Venture-Captain, Massachusetts—Boston aka Harley Quinn X|
There's an FAQ on scribing scrolls in PFS.
Players are welcome to exchange spells with each other during an adventure. They must still follow all the normal rules as put forth in the Core Rulebook and their class descriptions (for instance, an alchemist can scribe from a wizard, but not vice-versa) and they must not bog the session down.
Similarly, scrolls found during an adventure can be used to add spells to spellbooks and similar class features (such as a witch's familiar), using the normal rules for doing so. Scrolls used in this way during an adventure do not need to be purchased, but are still consumed as normal.
With either method, the GM should sign off on the spells gained (after witnessing successful skill checks) on affected players' chronicle sheets. All other methods of gaining new spells (such as by gaining a level or purchasing access to an NPC's spellbook) function as described in the Core Rulebook and relevant class descriptions.
In the rare instance of a wizard charging a fee for the privilege of copying spells from their spellbooks, this fee is equal to half the cost to write the spell into a spellbook (see Writing a New Spell into a Spellbook). Rare and unique spells do not change the fee in PFS.
So you can purchase from NPCs in the Society's libraries, but you'd need to pay the scribing costs and give them a 50% tip.