I use getty images for a lot of my corporate work. Shutterstock is another. I think Sony has one now. You have to pay for their stuff though. There are some public domain stock sites which would mean they are clear to use. Just do a search for them there's a bunch of both.
Most places you sign up and then you can get access to a low res version with a watermark. If you want to buy it, you pay them and they give you access to the highest quality they have of it. Prices vary depending on the topic and rarity. If you find something that appears to be p.d make sure you read the fine print and make sure it is okay for you to use it.