Rick, the NeoAir is rated to freezing, and I started getting cold on it in the upper 30's. You must be really warm-blooded!

Jim, to replace a down mat I'd suggest another Stephenson, an Exped Down Mat, or contact KookaBay (use the "contact" page on their web site) about their custom down mats.

If your current mat is OK except for the valve, I would think that replacing the valve, if possible, would be far less expensive than a new mat!
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey