Clickity-clack for southpaws...
Perhaps the easiest route to go is the Remington 870 in left hand. I have two with one being 12 ga and the other 20 ga. However, don't overlook the Ithaca 37 shotgun which features bottom ejection and feeding. Apart from the safety, the Ithaca doesn't care what hand you shoot it with and if you get her an older used one which doesn't have a disconnector, it will shoot as fast as she can pump it.
BTW, don't know any Southpaws who have encountered any problems using a wrong sided shotgun. We shoot 'em just fine but there's no harm in pampering ourselves with portside shotguns.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!