I bought my extra 10 rounders, back when they were talking 10 round max mags. thought they would be hard to get once the demand got great. I purchased mine from:
Ivanhoe's Warehouse Outlet
Box 937
Nutting Lake, MA 01865
888-811-2224
I bought them for about 1/2 of manufacturer's (Ruger)price and the were factory packed magazines, not knock-offs.