I'm not familiar with Fall of Plaguestone, but the best way to determine which pawns you'll need is to see which monsters are called for and see their source.
At a minimum, I'd suggest the Bestiary Boxed Set as it will have all the "core" monsters. "Monster Codex" and "Enemy Encounters" will give you multiples of some more common monsters. For PCs and NPCs, "NPC Codex," "Inner Sea Pawn Box," and "Heroes and Villains" are good resources.
Be warned, some of these are out of print, but you can still get the PDF and create your own. I've had to do that. I posted a thread showing my technique here. Some others commented with their techniques as well.
Oh, check out "Traps & Treasure" and "Dungeon Decor" for scenery pawns.