Moik
You need to think about this. Why would real estate agencies put their porperties on your site opposed to the millions of real estate sites out there. If this is a new site you'll have little or no traffic, which is no incentive. You will have to offer to do it for free, if you want to get them. Even at that , you might have to put them up yourself. Also they are going to want to know what keywords you rank for? i.e. if I'm selling australian property, I'd look to put my properties that ranks for "Australia Property". Focus on the SEO of the site, so you have something to offer. Look for property forums, where you can market the site. DON'T SPAM the forums, but maybe higher some forum posters to offer advice on properties with a link to your site in the sig. Look into article marketing for the site also, it will give you an authority impression on the real estate industry. Offer high authroity property related sites , free editorials, including a link to your site. Get your site reviewed by property bloggers. Try to partner with local press media providing articles on the property market. Locate your unique selling points and exploit them.
Depending on your industry, some overseas property sites, offer xml feeds of properties which you could use, some charge though