Call TRE to see if they can still get the probe they used to sell. I run one with my Defi gauge and its the best probe I've ever used. TRE took it off the website though so I'm afraid they may not sell it anymore
If all else fails you could buy a replacement from Defi for way too much or one from Prosport.
http://prosportgauges.com/exhuast-ga...ure-probe.aspx