To find the correct parts for a door, first determine what type of door you have. Doors with automatic closures require different parts than those that close manually. Storm and screen doors often have closure mechanisms and springs that are not found on regular interior and exterior doors. Your local home improvement store may be able to offer advice on what parts they have available to fit your door.
Doors can have many different types of locking mechanisms. If you can find out the name of the manufacturer of your door is or find a model number you can use this information to find manuals and parts lists. Sometimes parts can be found at a hardware store but it can be a challenge to find the right screw or bold when faced with thousands. The manufacturer is more likely to be able to provide you with the correct parts.
Door knobs and deadbolt locks are items that are easy to find replacements and parts for because they are sold at almost all hardware stores. For historic doors, there are specialty hardware retailers, such as Signature Hardware or House of Antique Hardware, that can provide accurate reproductions of hardware from bygone eras.