Here are some places online to find US city directories:

• List of directories on Fold3.com—fee based.
• View the Ancestry.com page and browse–fee based.
• Search at http://books.google.com –free
  • try “city state directory year”
• Search at http://www.archive.org– free
  • use the advanced search options as above
• Search at http://www.hathitrust.org/ – free
  • click on “catalog” and do a title search as above
• Local library websites may have some
• Local or regional genealogical societies may have some

Feel free to enter your favorite search sites for these in the comments.