I got it at a electronics store here (Jaycar for the locals) a few years ago. You press the button and the display changes to fahrenheit (in hindsight, I should have took a pic of that too!). It was just on the counter with other junk no one would ever buy! Just checked their website and they don't list it as a stock item.
Never tested it for real accuracy, but it seems close enough. I just use it for a rough guide to make sure I don't wash my film with too cold or hot water.
A quick google of "inline water thermometer" turns up some fancier devices. Looks like they are used for home brewing beer. A worthy hobby!