You have five good options for purchasing your doors:
Home Improvement Stores
Stores like Home Depot and Lowes have a good selection of inexpensive to moderately priced doors. They also can either provide or recommend local contractors to install their products.
Lumberyards
Lumberyards usually carry doors, but the selection is variable. Lumberyards are more likely to have a selection of unfinished doors.
Door Specialty Stores and Showrooms
If you live in a metropolitan area, there are likely door specialty shops and showrooms. These shops tend to sell medium to high priced doors and hardware.
Millwork Shops
If you cannot find a pre-manufactured door to meet your needs, you can take your specifications to a local millwork shop and have your door custom made. Custom made doors are also available from some online door etailers.
Online Door Etailers
There are a number of companies now selling doors online. Some of these companies sell locks, as well. These companies often have a wide selection, good service, and knowledgeable phone representatives. DoorSuperstore.com and Exterior-Doors.com are two of these door etailers.