I use Kijiji and this site for online ads. I have also put ads in the local papers, advertised via Facebook (through my status updates or by creating a group and inviting people), and when all else fails, I use old-fashioned paper flyers. Put them at the grocery store, or local big box stores (most of them have an ad space), in the library, at local hot spots for moms/kids (ie; playspaces, or swimming pools).
And don't forget word of mouth!! Let everyone you know, know that you have space(s) available.
Good luck!