If you can find M-14/M1-A magazines, Armalite will convert them to AR-10 magazines- though I heard here that the mags you send them aren't necessarily the mags they send you back, they might send you "equivalent" magazines which were M1A mags they already had and converted them for future buyers.
These guys are a truly excellent source of M14/M1A mags in terms of quality, price and customer service, and have been mentioned here many times as a source of M-14/M1-A magazines.
Go here and click on "Product Info"
www.coledistributing.com