That would depend on your niche and kind of products you are selling.
For internet marketing for example, you can check with popular internet marketing forums that accepts promotinal offers. You can then specify some days for of giving out free products in exchange for reviews. Each forum and website has its own rules and regulations. Check up with them before asking for reviews.
Usually, to force your clients to give you a positive review, you would need to give them the product for free! letting them buy the product then offering them bonuses for reviews is a much harder way.
If you can specify what kind of niche you are working on, it would be much easier to help you out!
All the Best!