It looks like you're getting the AR15 platform mixed up with the M1A (Springfield SOCOM) platform.I have a friend that's in my National Guard Unit that stresses to just buy the parts and build one yourself. Buying the complete gun outright is a waste of money. You could buy the parts twice and have to guns for the price of one complete gun.
I'm not familiar with either brand, but I'd look for SOCOM parts as its seems to be more popular, or at least in Idaho it is.
You could try Gunbroker.com for the parts, just type in "ar-15 upper" and "ar-15 lower".