Try researching RTDs, thermocouples, thermistors, bimetallic thermometers, and IC temperature sensors. All are available with wide differences in characteristics from a variety of suppliers. Some of the above are active or passive electric/electronic devices, others require no electrical input. You could check with electrical supply stores, electronics stores, scientific equipment suppliers, etc. I don't want to specify any source by name, as I don't feel these forums should be used to promote any specific company over any other. Besides, from what I listed above, you should have no problem finding sources on the Internet. E-mail me if you need more information on the advantages and disadvantages of the types of sensors I listed above. Best of luck.