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I bought one from Academy Sports for $1.99. Pretty basic thermometer but it had a 12" stem. I punched it through a cork and adjust it to reach the top rack of my Char Broil Double Chef through the top vent. For that price, I'll replace it every year if I have to.
I don't recall if it specifically was a deep fryer thermo, but it looks almost exactly like the one in DDave's post. It has the same clip and red target pointer on it.
I bought 4 of these 1 for each wall on my smokehouse. I put them at various heights so that I know what heat stages are in the different heights of my smoker. They work really well and you can calibrate them by turning the rear nut to 212 degs in boiling water. I also use a maverick ET- 73 for my internals and as a check for my stems. I highly recommend these for the price. http://www.sausagemaker.com/4950312stemthermometer.aspx
Almost store that carries a decent amount of grilling/outdoor stuff should have at least a 12" deep fry thermo. I recently bought one at Menards, checked calibration in boiling water and it was less than 5 degrees off. It did seem to go funky up around 400 degrees; using two thermos they both stayed very close up to 400 then the deep fry thermo climbed like a rocket while the other one didn't after 400. I'll just use the deep fry thermo for my smoke chamber thermo because I normally smoke around 225-250 anyways so it should be fairly close at that level.