I have a US publication number US6808849 for a granted patent. What do I have to do to find the related published application number US2003064291?

One obvious way is to make a simple bibliographic retrieval for patent US6808849 while setting the constituent on "full cycle":

You will get the bibliographic data for all the publication steps relating to this invention, including the number of the published application US2003064291.

An alternative is to make an INPADOC family request. The response will contain basic information (publication, application and priority numbers ) for all the family members, including the various publication steps for that invention: