That's a regular square-taper double crank that isn't low-profile in design, so a left arm from any same-length, standard-profile, price-point Japanese crankset would probably work.
In other words, it's a very common item to find, though not too many people go to the trouble of listing their orphaned individual crankarms for sale.
No doubt someone here has just the arm that you need, I'm sure that I do, and you shouldn't have to pay much more than $20 shipped for such an item.
You could also find such a part at any bike co-op that might be in a city near you. Boxes of them, for cheap.

Upgrading the whole crankset might also be inexpensive, though the front derailer would need to be readjusted following such replacement, and the right spindle length of the bottom bracket becomes a more-sensitive issue at that point. But if you are happy with your current gear ratios and your bottom bracket spindle bearings are still good then just a left arm is the simpler repair.