Ebay - search for Tel-Tru. They have a lot of options. I got the one for my WSM (the old model before they came with a dome thermometer) for $25 and it was brand new. The Tel-Tru models are commercial grade.
Tel-Tru would be my first choice as well they make lots of different ones. Personally I'd go with a Maverick Digital or similar and thread the cable and probe into it before I went with a Fryer Thermometer