The illustration from Bouteaux in the previous post is from Botanicus, the rare book collection of the Missouri Botanical Garden, which is now fully scanned (page by beautiful page) and available online for either viewing or download! Many thanks to the MBG, as sites like this are an absolute dream for garden historians...rare volumes are often only available in a few libraries, and traveling to them just to see a page or two is difficult. The Bouteaux volume is one of their only garden design holdings, but there is a wealth of botanical volumes, many with beautiful illustrations, including tomes on Himalayan plants, orchids, and medicinal plants. Search the entire catalog at http://www.botanicus.org/, and also take a look at their older site, which has helpful thumbnail images. I wish they had preserved this feature in the new site, but it's a small complaint.
P.S. I've just noticed that you can also purchase prints!