You've got the right idea and terminology . The combiner is a plate made of zinc selenide coated to transmit the co2 beam thru with little loss while completely reflecting the red diode or HeNe beam coming in at 90º, therefore combining the two into one beam.
Just do a search for 'znse combiner' and you'll find several sources.
One note, if your on a tight budget ZnSe optics tend to be expensive. A common less expensive technique is to use a small regular mirror mounted to a solenoid actuated shutter that moves in and out of the beam path. The only difference in doing it this way is that you don't get the red dot while the laser is actually working (which is useless anyway!).
Laser.Tech