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If your American Express card is issued in the USA you can use the Points Advance feature to borrow Membership Rewards points that you have not yet earned. If you have a Green or Gold account you can borrow 15,000 points, if you have a Platinum or Centurion you can borrow 60,000. You have 12 months in which to earn sufficient points to repay the advance.
Alternately, American Express will sell you points at $25 per thousand. There is no place else that you can buy Membership Rewards points.
Membership Rewards is discussed here:
http://www.flyertalk.com/forum/ameri...p-rewards-410/