1. Driving for dollars/walking the neighborhood for FSBOs and For Rents. Pick a farm area close to where you live and or work. Drive through a different side streets every time you drive to and from work or run errands. You can also walk the neighbor hood.
My local dollar store has 3x5 orange post it notes. It looks like an official notification from a distance. You can use those to leave your contact info if your walking the neighborhood and come across FSBOs or for rents.
2. Turn you car into a mobile billboard. $10 at vista print gets you a car magnet.
You can also get the kind of window markers people use to write just married or for sale on their car windows. Instead you write I buy houses and your number. Just make sure you're not blocking your view.
Get a big poster board $1 - 2. ( I found some at my local dollar store) , write on it I buy houses and your number. When you go to the mall, movies etc, back your car into the space. Use the poster board as a sunshade. Advertise your business and keep the inside of you car cool.
3. Networking at your local REIA. $10 at vistaprint gets 250 business cards.
4. Advertise on Craigslist and other online classified sites.
5. Flyers - $5 for 600 sheets of printer paper at wallmart, $1- 2 for markers, $1for thumbtacks. Put up flyers at super markets, laundry mats, etc.
If you do all five of the above ideas, it should cost you about $30 or less.