"Le vélo de Tati” - Jacques Tati by Robert Doisneau Does anyone know where i can purchase this iconic poster? I've searched the most obvious websites. 

thanks
This is a bad answer, but its been six months and noone else has a better one.

For personal use only AND given there appears to be no other way, you could either

1)  recreate it for yourself and take your own photograph, 

2)  Scale this one as big as you reasonably can and then manually retouch it.     https://s-media-cache-ak0.pinimg.com/736x/c5/2a/42/c52a428f7765eaaaa7ccbc1151b2f4f5.jpg

Both would probably be derivative works, and as long as you're not claiming it as your own original work, then there's little chance of someone accusing you of stealing the image.   

Remember to give credit to the original artist.