Try :Primary Arms,
Joebob Outfitters,
Cheaper Than Dirt,
Magpul store,
Brownells,
Ebay.
If you search any of these stores for parts you will yield the results you desire, just bear in mind that there are slight colour variations between different companies. I would say that the colour on that particular AR15 is not Flat Dark Earth but Tan or Coyote Tan, FDE is closer to Olive Drab than it is closer to Tan.
Confused yet