One of the things you can do is take small labels and use them for signs. I have a Dunkin' Donut building w/ a sign cut from a Munchkins box. I have a Jameson's Whiskey billboard cut from the bottle's box I brought back from Ireland. Some Nabisco brands to go on the side of buildings, etc.
You can also get very small sticker letters, set in either Z or N scale for signs.