From the sounds of it a new mask would be a start. Some suppliers will let you buy and try masks. If they don't fit, you can return or exchange them. I'm not sure if you need a prescription for masks or not.
Another good spot for used gear is Craig's List. You will have to know what you are buying. I think the 2 most popular brands are Philips Respironics and Resmed. In the past year they have both released a "10" series line of machines. That makes their "9" series less valuable, although they are great units that have excellent data reporting, humidifiers and function perfectly.
Tip: When you use a humidifier you should get the heated hose to prevent moisture, or as we call it "rain out".