I wrote a blog on how to find lease-option properties and others where the owner is willing to finance or where the owner doesn't need all of his/her cash out immediately: http://www.trulia.com/blog/don_tepper/2010/03/how_you_can_fi
As for needing 20% down--no way.
One of my suggestions in my blog is to search on Craigslist. Here's a Craigslist link for owner-financed properties in Chicago: http://chicago.craigslist.org/search/rea?query=owner+financi
And here's a link to lease-option properties: http://chicago.craigslist.org/search/rea?query=lease+option&
The down payment (in the case of straight owner financing) or the option fee (in the case of a lease option) is fully negotiable.
Hope that helps.
Americorp Real Estate
Brokers Associate, e-PRO