Ignitors on gas furnaces can be replaced. The particular method of replacement and type of ignitor depends on the model and manufacturer of the furnace.
The DoItYourself.com and HVAC-for-Beginners.com websites offer guides on how to replace the part. Two common types of ignitors are spark ignitors and hot surface ignitors. Hot surface ignitors are more common in newer gas furnaces. If the gas furnace uses a hot surface ignitor, silicon nitride ignitors last about six to 10 years, or twice as long as those made from silicon carbide. As of March 2015, both Amazon.com and Sears sell ignitors on their websites.