While I am getting a bit tired of stock images (first only the cliche ones, and now actually almost every image that is not real), I find new inspiration in textures of real-world materials (there are lots of these on stock image sites). See the ad below for the folding bike (via Ads of the World).
Do not forget to compress your images before saving. High-resolution textures can consume a lot of disk space on your computer and as an email attachment.