why buy one if you can print one?
for anyone who still doesn´t know this comes very handy:
Free Lens Hoods
Printable PDF documents to make your own lens hoods
This site contains PDF documents that can be printed and cut out to make lens hoods for a variety of 35mm and digital SLR camera lenses. Lens hoods give better quality photographs by reducing flare caused by nonimageforming light.