GE 3 way Switches are no longer available. What is a good alternative?

GE is updating their model line to zwave plus. The new model should be available shortly on Amazon if they aren’t now. You can also buy them from the manufacturer site (the GE zwave switches are made by Jasco).

http://www.ezzwave.com

As far as mixing switches, no, you cannot do that. You have to use an auxiliary switch which was specifically designed to work with the master.