512.
Look at the various magazine suppliers who sell the gray Teflon mags (Labelle's)
Short of the HK High reliability G II mags, these Teflon mags are the shit
They are very durable, feature the latest followers, and enjoy a great reputation among serious shooters.
Many dealers sell them, although it can be a timing thing. One day- yes. The next day - gone, but don't be discouraged.
Next, I would go with the Colt 30's. Not because they are Colt's, but because Colt's selects the best bodies, springs, and followers for their products
SAW sells these and are available pretty cheap right now.
HTH