Many airlines will allow you to buy 5000-15,000 extra miles per year if you are a few short from being able to redeem for a free ticket. The cost ranges from x - y, which may seem expensive but if you are just a few short it may be well worth it when compared with actually buying a ticket.
Below are links to the various airlines that sell miles ....
Alaska Airlines $0.025 per mile max: 30,000 miles
American Airlines $0.0275-$0.025 per mile max: 40,000 miles
British Airways 0.03-$0.0156 pounds per mile max: 25,000 miles
Delta $0.0275 per mile + 7% max: 30,000 miles
Frontier Airlines $0.028 - $0.025 per mile max: 15,000 miles
Midwest Airlines $0.02938 - $0.022 per mile max: 15,000 miles
Northwest Airlines $0.038 - $0.0296 per mile max: 15,000 miles
US Airways $0.035 per mileplus $25 max: 50,000 miles
Virgin Atlantic $0.035 per mileplus $25 max: 50,000 miles