Ethically, its not a good idea. The fact is that you're making a mock-up and the possible copyright owners will never see the use. The reason I suggest not to is because if a client does end up with a digital copy for proofing, they might end up distributing it among departments to get opinions. That might be the cause of a leak that could expose your illegal use of the photo. Of course you'd have a better chance of finding a needle in a haystack than being "busted" for using that photo, but why risk it?

You have two options:

1. Use a royalty-free image that is also free. Here is a link for 50 sites to find free stock images. Each site has different rules, read them.

2. Use copyrighted images with a watermark. That's right, grab the ones right off of Shutterstock, Corbis or any other big site. Deal with the ugly watermark and tell your client it is only there until real images are put in.

Thanks SurfPark, unfortunately for me - it seems you are right as this is a real scenario im now dealing with.

You can mock with a watermarked royalty free image. Just swap it in for the real thing later. Seems legit to me. I do it all the time - don't want to purchase an image before the client has approved the purchase.

Is there nothing in stock photo sites that are similar to what you have found on the net?

The problem with using images you "found on the net" for mock ups is that the client wants THAT ONE IMAGE and you're in a world of hurt trying to secure the rights or getting something with a printable resolution. Been there, done that.
Use imagery that you know you can secure rights for.