I've ordered from deraailleurhanger.com and was pleased. Their site recommends:
"To make a purchase:
Choose your derailleur hanger carefully. The photograph is probably the most reliable method. Also take into consideration the models said to use a particular hanger. Sometimes customers do not remember the year of manufacture of their bike, or even which model they have. Also, some bicycle models used two different derailleur hangers within one year's production. Please rely on the photo as the best means to identify which hanger you should order."
Or call and talk to them.