Steve at
www.adco.com has always had the best prices on lowers that I found an RRA nat. match (nat. natch= good trigger)
last time I checked they were 302.00, delivered. that was when they were 350.00+ S&H from RRA.
No AR-10 lowers are not the same, the housing is bigger to accept the 308 length mag, but all the parts are interchangeable except for a few small ones, as I stated in above post.
Not sure about armalite but I'm certain Eagle arms (sister co. to armalite) sells lowers separately.
RR
Will pm with more info