Colt has some units in circulation with milspec fire control parts, no sear block, etc.
Bushmaster has 14.5" and 20" 1/7 twist milspec barrels.
DPMS has milspec 1/7 chrome lined barrels but the front sights are cast, not forged.
The only companies I know of with 4150 steel barrels are Bushmaster, Colt, DPMS.
There is no such thing as a "milspec AR15 lower" as none will have the sear hole. Only Colt uses the correct extension tube on carbines.
However if you want an AR15 with aftermarket stock and folding front sight there are some viable combinations in there.
The only things you want to really check are:
Will it fit a RDIAS and any after market trigger I want?
Does it have a chrome lined, 1/7 twist, 4150 steel barrel?
Will the stock I want fit on the lower reciever extension or must I buy a new one?
Is my bolt/carrier as close to milspec as possible? (I suggest Bushmaster or Colt M16 units)
Are the 1913 rails on my upper to propper spec?