Tufte sells it.
I haven’t found a site that displays it without a ton of junk. Just search for “Napoleon’s March by Minard”.
Wikipedia has it: https://en.wikipedia.org/wiki/File:Minard.png
A higher quality version is available from the French national library: https://gallica.bnf.fr/ark:/12148/btv1b52504201x
Tufte sells it.
I haven’t found a site that displays it without a ton of junk. Just search for “Napoleon’s March by Minard”.