Where can I guy a Moen Scald Guard valve?

Lowe's, Home Dept, any number of hardware and plumbing supply places that sell Moen brand stuff.

Contact Moen they should send you one free.

You mean entire valve or just the anti-scald device that fits under the handle ?

Have your valve model number ready and Call Moen at 800-289-6636 explain your problem to them they will send you any part you need for free, good luck.

John