They had been out of stock everywhere for a while. I just got a notification last week that Shield Arms has them in stock again. I haven't seen them pop up on gunmagwarehouse.com or any other major online retailer yet though. $42... I thought it was overpriced when I spent $28 on mine originally...
There is a gun shop in Vienna, VA where I bought mine. They only had a couple of these in stock so you may want to call ahead before you drive over there.
Give these guys a call, I bought two S15 mags from them at the last Chantilly show.
$42 each, same price as Shield sells hem for on their website.
The other two venders that I saw that had them wanted $49 each.
They are located in Chantilly.