To post ad for free many company you can find at search engine, on my mine the free ad marketplace is adwords, Why google adwords ?
Here i will tell you,
Adwords is the big of business ads, with adwords you can publish your product to the worlds, Because adwords is the right business ads and the first ad marketplace that support any languange.
If you do not know how to post ad, let's type on your browser
google.com/adwords, here you be explain about how to put ads and you get free, but it's work when you must to obey the terms of policy of their company.
You must pay it after you get a money from this ad marketplace, If you want post ad with the keyword cheap to you.
Oke fine, do it now before the keyword so expansive !