I would strongly advise you not buy a mercury thermometer, glass sure, but not with mercury.
You do not want to ever deal with breaking a mercury thermometer in your house.
You should read what OSHA says to do for clean up if you do, it is nightmare, not to mention the danger of it
and If you are concerned about heavy metals you might consider going to the Cutler Chelation Forum and do a hair test and find out if you have a metal problem. There seems to be a potential for Iodine to stir up heavy metals in the body.