Airlines like to do this. They sell their miles for more than the statistically expected cost of redeeming them for awards. For some, selling miles to partners is a major profit center.

Each airline has its own program. There's no central "get any airline's miles here" office. That's why only the largest affiliates, such as major hotel chains or car rental companies, offer customers a choice of airlines on which to get miles.

Contact the individual airline. If you don't have a name or a title within their frequent flyer program, any customer service agent should be able to aim you in the right general direction. It may take you two or three pass-alongs to get to the right individual.

Expect to pay in the general vicinity of 2¢ per mile. The cost goes down as the number of miles you want goes up. Figures as low as 1.6¢ have been mentioned for Hertz. At the level you're probably talking about, your cost per mile will be at the high end of the range, not much less than a member would pay to buy the same miles from the airline (if it's one of the airlines that allows individuals to buy miles).