In my current campaign, I'm drawing from a number of sources:
Monster books
I've got my own master list of monsters from all of the sourcebooks that I have, including both WotC and 3PP stuff. I've also made notes for particular monsters where I've had specific encounter ideas but not had the opportunity to use them. I'm especially a fan of templates because of their versatility. As a particular call-out, the
Tomes of Horror I-III from Necromancer Games, the
Monsternomicons from Privateer Press (probably my favourites by way of flavour and mix) and the
Advanced Bestiary by Green Ronin for it's versatility.
WotC books
They're all available, with the caveat that similar feats/abilities/classes that have been retooled in Pathfinder trump the 3.x source. I've also outright banned a handful of classes, feats, spells etc.
3rd party publishers
I've got a lot of 3rd party stuff, both in print and PDF. At the very least, they often provide inspiration.
- Poisoncraft (Blue Devil Games)
- Book of Iron Might (Malhavoc)
- Book of Roguish Luck (Malhavoc)
- Book of Hallowed Might (Malhavoc)
- Book of Eldritch Might (Malhavoc)
- Arcana Evolved (Malhavoc)
- Requiem for a God (Malhavoc)
- When the Sky Falls (Malhavoc)
- Chaositech (Malhavoc)
- Beyond Countless Doorways (Malhavoc - great book)
- Iron Kingdoms Character Guide (Privateer Press)
- Iron Kingdoms World Guide (Privateer Press)
- Liber Mechanika (Privateer Press)
- Sunken Empires (Open Design)
- Book of Drakes (Open Design)
- Steam and Brass (Open Design)
- Corsair (Adamant Entertainment)
- Steam and Steel (ENPublishing)
- Aerial Adventure Guide: SKy Captain's Handbook (Goodman Games)
Ultimately, I draw from anything that makes sense at the time by way of flavour or by being fit-for-purpose.