Stocksy has some great photos that I use on occasion.

Unsplash is my go to. Especially with the integration with Craft plugin within sketch, makes inserting images easy. (and legal ;))

for premium, thinkstock (owned by getty...we have a subscription) for everything else, i use a few like Pixabay, morguefile, freeimages.com, sometimes compfight...

We have an in-house photographer. He's a beast.

Are you serious about "just steal them"? I hope not. Usual I can find something usable at one of the many free stock sources, otherwise I use Getty images for premium stock.