SupplySpy is described as 'software uses a data-driven methodology to help Amazon sellers find new products and suppliers, track SKU price fluctuations and profitability, and gain valuable insights into data that affects their business'. There are five alternatives to SupplySpy, not only websites but also apps for a variety of platforms, including Mac, Windows, Linux and Google Chrome. The best alternative is Jungle Scout. It's not free, so if you're looking for a free alternative, you could try AmzChart or Upwin club. Other great sites and apps similar to SupplySpy are SellerApp (Paid) and AMZScout (Paid).
AmzChart is an Amazon product research and BSR chart tool. BSR stands for best sellers rank and it's a ranking that Amazon gives to the products based on a number of factors such as rating and reviews, sales velocity, and other methods that they never disclosed publicly.