My brother and me got 6 new in the wrap Winchester made M14 mags from [email protected] He doesn't have a website but just email him for what your looking for. He does a lot of sales and is good to deal with. He has a banner add that pops up on www.battlerifles.com , where he is also a moderator.
My vote would go to Cole's due to personal good experience with them or Brian's due to extremely good reputation and recommendations. Anywhere else is a crap shoot as to whether you get what you pay for. You may save a few pennies but it may turn around and bite you in the butt when it comes to reliability and/or quality/condition.