Yes, any auto part store will carry di-electric grease. Look at the shelf with all the RTV Silicone Tubes, sealants and glues, there is usually small tubes of di-electric grease for changing spark plugs.
Most also have a cheap jobber pack on a rack at the check out counter, along with jobber packs of anti-seize, brake grease, etc.
Also pick up a can of electric contact cleaner as well. You've got to clean the body of the sensor as well, with soap and water, do that with the connector connected so you don't get water in the connectors/contacts, then use the spray can of contact cleaner on the connectors/contacts. Look for corrosion in the contacts as well, and try to scrape/brush it off as well as you can.