If you want a 1956 wine just to say you drank one, then Bordeaux is your best bet. They are relatively cheap and you might actually find one that is drinkable (note the recent TN on the 1956 Haut-Brion). I've had the Haut-Brion and also the Latour, and both are passable wines at best, of interest mainly due to their age and the dreadful reputation of 1956 Bordeaux.
If you want a 1956 wine that is more than just drinkable, the (surprising) place to look would be California. Charles Krug Vintage Selection and Louis Martini Special Selection were both good, bordering on great wines when I last tried them in 2001. Both will be expensive because they are the only decent wines made anywhere in the world in 1956!
Look for Bordeaux in 1957, specifically Graves. They will be expensive, but in the case of La Mission Haut-Brion and Haut-Brion they are probably worth it.