Dover Publications produces books full of clip art, patterns and historical drawings to help inspire your artwork. Some of the books even come with a CD of the images so you can manipulate them on your computer as well. I just purchased Flowers: Bursting with superior botanical illustrations, this gorgeous collection features more than 200 blossoms and bouquets, hand-picked from the rarest sources.
Since I was in Thailand last year I wanted some more references for Thai inspiration.
And I bought Thai Decorative Designs.