where do most people get them to use in their ads and landing pages? adult and non adult
I don't want to google/facebook and use something without permission.
We license stock images.
I use a mixture of stock, custom-created (either CGI or physical shoot) and Creative Commons images.
Depending on what you're looking for, Flickr's Creative Commons options are pretty awesome, and there is an ever-increasing choice of other CC image sites too (Wikimedia FTW). Just a) make sure to stay compliant with the right license, and b) TinEye search to check that the image really is CC-licensed.
If you're putting out a high volume of campaigns, stock photography sites can be a pretty good bargain - most of them offer a "subscription" option where you can download 20-ish images a day for about $200 a month. That's a LOT of images to test.