Ace Hardware, Lowes, Menards.
just get one that is made for higher temps
i think that DIY setup is cheesy, utilizing a poultry temp guage lol. and still you are reading temps from the radiator, not temps in the exit hose out of the motor
i would think when you are reading temps coming out of the overflow spout, that its too late...
plus, being in the overflow path, that would require a constant flow wouldnt it? to get accurate readings all the time.