There's an example of a luscious German one in Fisherman's Cottage, Deal, where I stayed a few months ago. Get your hands on something similar, but in a clean and serene pale wood frame, from one of my favourite shops, Present and Correct (heaven if you're a stationery nerd, too).
Cherry chart, £80
This original botanical classroom chart from 60s France is in mint condition and features lovely "cerisier" (yes, cherry trees) diagrams. It measures 36" x 27"
Flowers Chart, £75
Another chart from the same set, but featuring flower parts. The measurements are also. 36" x 27"
Who knew the RHS sold prints? Not me, until last week... Relevantly, they do all sorts of botanical prints, such as these beautiful pear types, a print of an original 1771 engraving by Jakob Folkema after an original by Johann Hermann Knoop. The price is for an unframed A4 print, though you can get it bigger, too. They'll also frame it for you, and it comes as a canvas print, too (though, personally, I think the latter would cheapen the look).
This print of what was originally one of a series of 25 drawings by the 18th century French artist, Pierre Turpin. Underneath the main drawing are uncoloured dissections of the flower and seeds. As above, it comes unframed, A4 for this price, but larger sizes and frame/canvas options are available too.
And why keep it traditional? This book plate of graphical butterflies is from 20th century paris. It's from the utterly brilliant V&A shop, and is A4 (but, as with the RHS prints, comes bigger etc.)
And it doesn't even need to involve nature. Love these charts of china teacups, above left, and types of cake, right. Pretty.
And for a purely digital, and thoroughly modern, version of nice groups of things laid out pleasingly, have a browse at Things Organized Neatly.