Most HK's are ambi. Check out the Grand Power K series. STI used to import them as the GP5 and GP6 and felt really good in the hand.
I'm a lefty as well. and all 3 pistols I own are 1911's with ambi safety. I have grown accustomed to using my left index finger to manipulate the mag release. In fact, some "Righties" will do just that to their 1911's and switch the mag release to the right side for competition and easier manipulation. It keeps the hand in a firm grip without having to loosen it to reach the mag release w/ your thumb. As for the slide release, I hardly ever use it except when trying out the trigger reset which is a bit akward. Just remember to take the mag out and you wont have any worries about slide lock. I mostly sling shot the slide for first round chambering.
Same can be said about Sig Sauer's and their decocker. I was disappointed that they don't taylor to lefties but decocker manipulation with my index finger is a breeze.