I suggest to take a look at Dropshipmate shopify app which eases your way to import products into your shopify store from not only the above mentioned sites but also 15+ supplier sites. And without any worries this app also helps you in importing product reviews and helps in daily auto sync of the products.
All the Best...
Maybe you can try to find the niche products from Amazon, Banggood, Alibaba etc..
And then, ShopMaster(shopmaster.com) can help you import the products from these sites and 23 more suppliers sites. Now you can get started with Free Starter Plan.