Davey,
You are going to want a half-mask air purifying respirator. This is simply a rubber/silicone facepiece that covers the mouth and nose, with straps that secure it behind the neck and over the head. You will need cartridges that filter out both organic vapors (the solvents you can smell when you paint) and particulates (also called "pigment"). Many hardware/DIY stores sell painters' respirators that come equipped with the correct cartridges and filters. If you don't find what you're looking for there, try www.labsafetysupply.com . They have a broad selection of respiratory protection devices. They also have a great staff that can steer you the right way and help you select what you need.
Having worn many devices over the years, I would recommend respirators by 3M, North, or MSA (Mine Safety Appliances). They all have models that provide a comfortable fit at a reasonable price. Expect to pay in the $20-$30 range for a good respirator. Advice: Stay away from e-bay. Many of the devices sold there are of questionable manufacturer, and have not been tested and certified by the National Institute of Occupational Safety and Health (NIOSH).
If you have specific questions regarding selection, fit, or maintenance, please don't hesitate to PM me. I'll be glad to help any way I can.
Gip