I used to love these so much as a kid, buying them from the Natural History Museum in LA. Even once as an adult I made a print using keys and wrote a love letter on the back. There's something inherently romantic about the monochromatic gradations, and that the paper has to be immersed in water and dried. I love that they are also called cyanotypes.
I pulled these from a shockingly small flickr pool, Sunprints.
Buy Sunprints here. They even have Super Sunprints.