Or buy stuff that's "royalty free" like the Dover books. Even then you can only use so many in your product at a time - the Dover stuff usually have all that info in the front or the back of the book.

The freebie texture sites I go to all have the usage rights on their site somewhere and I always look for it. If I don't find one, I won't use the images. Lack of any license info doesn't mean they're good to use. If it says "for personal or commercial use" you're ok. However, any site that has ANY creative commons usages I don't use either. They're not always clear which CC license. Those same freebies sites also have different purchasing plans for the amount of textures you'll think you'll need. If in doubt, purchase: most of the time those will give a very liberal license. Just be aware there are also unscrupulous texture sites too that swipe textures from other sites.

You could always find a site that creates and sells things for this purpose - such as vectors and photoshop files. They'll usually let you buy one at a time.

Laurie