eBay has mirrors available, as do many online stores. If you want to confirm that the mirror is the problem (because other mirrors may also be bad), you can run a test. You could set the laser to pulse for a set period of time (like two seconds), and burn the raw beam into plastic (or any material). Do this before, then after the mirror. Then, compare the two burns. If there is a significant difference then the mirror is deflecting or absorbing excessive energy and should be replaced. If you feel motivated, you could get a CO2 laser power meter probe and use it to measure actual wattage before and after the optic (and also check the tube's output power prior to the first mirror--to see if it needs replacement).