To answer gnu's question more specifically, I have seen left-handed simple tin snips, but it's possible the guy using them made his own. The couple of lefties I've seen had symmetrical handles, so in their case it was only the way the blades crossed that made them left-handed. Left handed (and left-handled) snips probably do exist as commercial items; but it's unlikely they would sell in sufficient volume to be in stock at any common retail tool outlets.
You might get help on it with a request for a special order through a hdw store that shows a decent tool selection. (That way the mfr guys will laugh at the dealer who wrote the order, instead of at you? Or you could use your real name on the order and just take the heat.)
My suggestion for the mid-sizes is that you pop in at Home Depot - or any other reasonably well-stocked hardware shop - and get one pair of Dutchmans (Compound Lever Aviation Metal Shears) in either right-cut or left-cut version, and after you try them out you'll want to decide which of your current set you want to move down to the bottom drawer where they're out of the way. The extra leverage, and some of the blade/jaw features, really do make them a lot more fun to use.
The Av Snips, incidentally, come in a whole bunch of "specials" like the one with double jaws that cuts a strip about 1/8" wide out of the middle of a piece of air conditioner duct - without significantly bending the adjacent metal on either side (so that you don't have to disassemble things to replace a bent area in a long run).