I have two sites that you'll find useful. The first is mentioned already by myronb. His link goes to the Home page and you can browse through different CD and DVD collections from there. The site also features a database driven, searchable clipart gallery
HERE. Purchasing single images from this site is done rather awkwardly through the
online store and orders are filled by email attachment.
The second site,
ExpressClipart.com is being setup to do exactly what you are seeking. While it doesn't appear to be open, it actually is fully functional and image licenses can be purchased and image files immediately downloaded. It does not, however, yet have the informational and instructional pages built so you kinda have to figure things out on your own until they are put online. It's designed to be intuitive though so it isn't difficult to figure out.
ExpressClipart.com is currently being loaded with images which takes a while. We currently have about 2,000 images online, each with 5 to 7 file format variations, and expect to exceed 20,000 images before the end of 2006. We will be featuring not only artwork that has been published in various CD collections but also new work not previously available from artists like our own Whit Fink and Stevo Designs.
Let me know if I may be of any assistance.