For any of the non airsoft specific parts try this site
http://www.tapco.com/ the have loads for the m16/m15/c7 yada yada yada series of rifles and the HK 614 is just a remade AR with a different blowback system so any sights should be interchangeable and there for work on the airsoft counterpart. Well the receiver and grip are slightly different, same rail type I think.