With no marketing budget, your best bet is to go to the source - the parents. Go to some local events, practices, etc. Take photos of whats going on and offer them up for free. Hand out your cards to the parents and get a good rapport going with them. As long as the images you offer up for free are good, the parents will choose someone they have interacted with, like, and trust over some random person that ran an ad. Network it!
If you have a decent portfolio, throw some ads on craigslist.