They post signs in places where there is high visibility and no cost:

In front of their own business
In front of neighboring businesses
Subway stations
School offices
Senior recreational facilities and retirement homes
College dormitories -- in the community area, hallways and bathrooms)
Fraternities and Sororities
Churches
Other local community clubs and organizations
Apartment buildings -- in laundry rooms)
Community activity centers
Grocery stores
Shopping malls
Car washes
Laundromats
Condominium complex party centers
Hotel and motel lobbies
Utility poles
Military cafeterias and recreation centers
Counters of public places
Meeting convention centers and rooms
Construction walls
Libraries
Union halls
Chambers of commerce
Medical or professional offices
Roller rinks and bowling alleys
Waiting rooms at auto repair and tire shops
Liquor and convenience stores
Company bulletin boards of friends and family
Tourist information centers
Highway rest stops
Banks
Factories
Their cars -- featuring a compelling sign, parked in a conspicuous place
On the fences outside a construction site

That's where guerrillas market for free. Where they market close to free?

College and high school newspapers and yearbooks
Classified services on the World Wide Web -- many are free
Classified services on commercial online services -- many are free
Local entertainment and tourist magazines
Community group and association newsletters
Small, local newspapers
Chamber of Commerce publications
Radio talk shows -- by appearing as a guest or calling in
A lingerie store owner stenciled small messages onto the sidewalks of New York, with provocative sayings like, "From here it looks like you could use some new underwear."
Projected at night onto the side of a building.
On your voice-mail message, but please keep it brief

By Jay Conrad Levinson, father of Guerrilla Marketing