It costs a butt load to have access to a drum scanner, which is the only kind that could handle that size. You really wouldn't need to use one of them unless your planning to publish them professionally and even then most people don't use that method anymore. Actually the pros just take photos of their pictures to record them. Thats what I do, thats what my Illustration teacher does, thats what several other professional illustrators do. Use a tripod, ether one you made or one you improvised or one you can afford. Thats what I do and my stuff comes out just fine.
http://beerchan.deviantart.com/ See?