Magpul is releasing receiver extension as well as buffer components such as buffer tube, springs and so on for ARs.
Link: http://www.magpul.com/stockcompletion
Receiver extension is made of 7075-T6 aluminum with Type III class 2 hard coat anodizing. It comes in 7-position, 10-position profile.
The buffer components available are,
Stainless Steel Carbine & Rifle Buffer Spring
AR15/M4 Carbine Buffers(H, H2, H3)
Heat-Treated Castle Nut
Receiver End Plate
ASAP
ASAP-QD
From: Magpul via Soldier Systems Daily