Need to give chance to people who are interested in writing for your niche markets. Generally i noticed webmasters create "Write for Us" page. So create write for us page and write your all terms and conditions there. Use your niche market keyword like "Write for Us - Share Ecommerce Article With Us". So, when people search about it like "Write for us 'Ecommerce'" then your page will appear and people will send you unique contents according to your niche market. A bit time taken process but you can get number of contents after few days or weeks. You can post in community like here that you are currently offering free guest posting for "Your niche markets" for free .... So may be some webmasters offer unique contents to get few backlinks from your website.