Our oil kenmore furnace was checked for the winter and the service man said it had a bad heat exchanger it is 8 yrs old and it is under warranty but we pay for labor do we have to have a sears repairman or could we have some one else do the work and are the parts available

The furnace was likely made by Inter City Products who carries the warranty. Any service person who deals with an ICP distributor should be able to get the heat exchanger for you.