Because things break down overtime. These links will help you fix your gun so you can go on spraying!
Sata Spray Guns
Look under the products tab, select the information that matches your setup, and under their downloads sections you will find their spare parts list
DeVilbiss and Binks Spray Guns
Binks was bought out by DeVilbiss so both their products information are listed together.
Select the "Parts" tab then select the from the left bar the information that matches your needs
Sharpe Spray Guns
Select "Manuals" button then the "Spray Gun Manuals" section. You will find your specific gun listed. The link will open in a pdf. file.
Iwata Spray Guns
Select the "products" side bar option underneath the logos, select the air gun or brush option, and find your piece of equipment listed. The parts guide will give you the breakdown of the gun.