Maybe do some fliers and simple business cards on the computer and post or place them at Vet's offices. I know when I go to my Vet there are several business cards and brochures for several different kinds of pet-related businesses from scooping poop to petsitting to arranging travel and boarding. You could also try advertising on your local Craigslist or free newspaper. There are also several pet-related websites where you could get the word out, although those may not be free. You could also 'hang out' where your anticipated clients congregate and spread the word that way.
_________________________
Well-behaved women rarely make history. - Laurel Thatcher Ulrich
http://ruthrainwater.wordpress.com/
http://newbeginningsgratitudejournal.wordpress.com/
http://sablewings.wordpress.com/