They've been OOS for several days in a row now at gunmagwarehouse, so maybe their supply finally ran out. I see 3-4 sets on gunbroker and you can find them on a few online retailers as well, which claim they are in stock but even if you put 1000 of them in your cart it shows that many being available, which makes me question that reliability of their online ordering system. Still, they are still "out there" at very good prices. If you ever want one of these, or if you want to buy/hold/sell-when-rare, now is prolly the last opportunity before all the cheap sets are gone.
I wish Magpul would respond to questions about these kits. I'd very much like to know approx how many were produced. I see the sand SL stock in various places, and the grip as well, but the handguard seems less common (outside the kit). It would be cool to know just how rare these are. They don't seem to have been widely released, unlike the sand mags.