There are a few around. I've helped a couple of guys find guns and wood in the States and get them up here. You are more likely to find left hand wood on a 682 than a 686 (at least in my experience). To add to Falconflyer's comments, the 686 is the basic "Chevy" of the Beretta line, the 682 is the "target" version and the 687 is the pretty version. Within the 680 series guns, all of the parts are interchangeable, sometimes without fitting (which should always be checked by someone qualified). Rib and comb heights vary, so, your comb and rib must match or you may see the back of the rib or all of the rib. The only exception is the 685, which is somewhat of an orphan. I've even seen 687 wood on a 682 receiver (looks funny without the side plates).
Many of the older 68X guns have more neutral stocks on them, which some leftys shoot well. You could purchase a right handed or neutral gun up here and probably find a lefty stock in the USA. Joel Etchen, as Falconflyer mentioned, would be my first place to look. Other big target shotgun dealers would also be worth checking as well as Wenig Gun Stocks. I've also had great luck on Trapshooters.com.
The main thing to keep in mind is that the rib and comb heights must match. For example, a sporting stock will not be high enough at the comb to be able to see the beads on an X Trap ribbed barrel, you will see the back of the rib.