The water temperature is going from very hot to very cold constantly do you know were I could buy a new mixer valve?

Mixer valves can be purchased in any professional plumbing supply store, like Ferguson Enterprises. Or go to web sites like Faucets, Kitchen Faucets, Bathroom Faucets, Sinks and Plumbing Fixtures at Faucet.com, select what you want and have it shipped to you...

Hope that helps / Milo