Where to buy an model airplane depends on the how much assistance you will need in choosing the right airplane for you and the support you require after the purchase.
If you do not belong to a club your local hobby shop is probably the best place to make your purchase. They can provide you with information that will aid your decision.
If you join a club then ask the advise of the other members on where to purchase.
Here is a list of websites were you can purchase a RC Airplane.