Something I've done before is to take original pictures of adds in magazines. This way, you can make them as big and detailed as you'd like. You need an SLR with manual focus preferably, and photoshop for color correction, cropping/perspective correction, and contrast etc. It's more complicated than an image search, but you have a lot more control, and it's kind of fun, especially if you have a lot of old mags laying around.
Here's a sample: