You can get them online for around $30. And you don't need the thermometer to check the level, just get the coolant temp to 80C. The purist will scream, but then they never bothered to check on the coefficient of thermal expansion for ATF.
When you change the oil you can measure the temp, if it makews your day, but I will bet you Svenska Kroner that when the tranny reads 80, your coolant temp reads 80 too.