ebay for that kind of stuff m8, as it's used for completely normal purposes like monitoring a conservatory
good mate o mine got a remote one so he can put the sensor in his grow and read it in the living room.
got it off ebay for a few quid.
i got a electronic one from poundland, guess how much it was
clock thermo & hygrometer, all in one, they do them in one of the shops as chrimbo gifts