I don't remember where but I found this website called Botanicus.org that has ancient botanical/biology books turned to PDFs and some of them have lovely illustrations! There are too many to check them all so I downloaded one that I knew had nice images. So here you are! The size is not huge but it was the biggest I could get out of the pdf.
I wanted to find two illustrations to hang above my bed and Ikea's posters seemed too expensive so these seem pretty good for the job!