Magpul has released a second generation version of their popular MBUS back-up sight. The folding front and rear sights looks sleeker and more refined. The AR style rear aperture and front post folding sights are injection molded and available in a number of colors. The sights will fit on any MIL-STD-1913 Picatinny/STANAG 4694 rail system, but it should be noted that because they’re plastic, they can’t be mounted to a front gas block. Probably because they melt. Magpul’s site warns “Front MBUS must be mounted to a railed fore end on the same plane as the receiver rail. Because of this the sight is not compatible with the MOE Hand Guard.”
If, for whatever reason, you still wanted the Gen 1 version of the MBUS, you can still buy it from Magpul’s website while supplies last, and you can probably expect local shops and gun shows to sell them for the next 2 to 10 years as they try to exhaust inventory without reducing prices.