I know Steve carries good ones at
Brewman, I think they are mostly Alla French made, because those are the brand his wholesaler stocks. When he started running Brewman we had a conversation about just this subject, as Steve is a chemical engineer so it wasn't a hard sell. I have found them excellent.
The reason I am recommending a glass lab thermometer is that they don't get flat batteries, wet contacts, internal corrosion, kinked wires... which anything electronic can do without your being able to tell. A glass thermometer either works or is broken, it doesn't change or drift - again without you knowing.
I have several sensors both analog and digital and regularly calibrate them against my reference thermometer, a pH meter that I calibrate before use, when I had a shop I has a 5, 9's kg mass to calibrate my scales - just part of doing it properly!
Like I said above you are only looking at somewhere near $20, I think the idea of posting one around would be a bit counterproductive, it would cost at least $10 per movement so with the $5 you put in, I cant see the saving being worth the effort. A NATA certified thermometer will cost about $400, it would probably be the same thermometer as the $20 one with a foil sticker - just get one from a reputable shop, make sure it isn't a $2 Chinese jobbie, no the don't bend or bounce but with care they last a life time.
Mark