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At most any RV shop.........or online at Amazon or E-Bay. Not hard to find. Just Google that and see who has the best price as it will vary. Did you test it first to be sure that was the problem? I am adding a link to the Dometic A/C manual for you and you can see the section relating to that. Hope this proves helpful here. Please get back to me if you need more later as all follow ups are included in our fee. http://bryantrv.com/docs2/docs/acservice.pdf
I am adding a link to one site for you............ http://getrvparts.com/Dometic-3313107025-Duo-Therm-Air-Conditioner-AC-Selector-Switch-3313107025.htm
Just checking back to see if the information I provided was helpful here or if you still need something additional from me?