A few hundred clip art images come with most word processors. The images run to holiday themes, sports, and sale banners. Some will be useful. On-line services and local computer bulletin boards offer similar clip art collections. The richest source of clip art is computer CD-ROM collections sold in software shops and through mail order.
Someone is sure to offer a bigger collection to compete on the store shelves.
I own or have looked at clip art sets totaling 100,000 images. Fewer than 1000 images are even potentially useful for my business, family, and personal interests. That is 1 but that is the way it goes with clip art. There are Macintosh versions as well. Most images in one set are included in the other.