Try the S&W M&P in .40 S&W, .45 ACP, or 9x19mm. The slide release is ambidextrous and the mag release reversable. Most lefties I know and those who have shot my M&P 40 use their index finger to release the mag and do it as quickly as I do with my thumb. Otherwise it is a lower cost option for ambi use than the H&K like the USP40 that I have as well. Mag release is reversable as well so you can make the LH your strong side with the thumb operation as well.