Stockholm's bus route map can be seen here.
If the above link is dead, try the following: visit sl.se/sv/reseinfo/, scroll down and click "Kartor över SL-trafiken", then click "Stockholms innerstad" for central Stockholm bus routes.
By the way, this post comes first when you google "Stockholm bus route map". And I keep revising this post because SL's website (sl.se) periodically changes its website structure and annuls the link. If you happen to be SL's website designer, please create the PERMALINK to bus route maps.