S&W did a great job of making the Shield available for purchase when they announced it...unusual for the gun industry...but sure wish they had the mags in stock as well.
It seems like S&W always has problems with providing enough magazine inventory. I remember when I bought my M&P22 last year shortly after introduction, it was like pulling hen's teeth to locate spare mags.
I normally use 44mag.com and Greg Cote, LLC depending on which has inventory and the lower total price.
I suspect that S&W didn't expect the Shield to be such a huge success, demand so quickly. It will be difficult to manufacture enough mags to fill the Shield orders asap, and then market/distribute additional mags separately.