Clipart ETC has the most amazing collection of old clipart. It is broken down in categories and is a great place to get inspiration. The clipart is free and in tif format, however there are some restrictions.
I'm not sure how I stumbled upon this site, but I love looking through it every once in awhile as it is like a clipart museum. It is absolutely fansinating to step back in time and then try and create something new from it. Have fun!