I'm a lefty as well. I find that for cost and size, the Glock 27 (chambered .40SW) works well. The mag release can easily be ported to your thumb side/left hand if it is more comfortable for you. I chose not to. Being a lefty you probably are also used to 'adjusting' to right handed tools. I use my left index (trigger) finger to release the magazine. It takes some getting used to but if that isn't comfortable, just switch the button to the other side. Get yourself to a gunshow you may find a used one. My thought is that the release button may be broken in more, save you some time and achy fingers!
Hope this helps!