Some products will not have suppliers that are already in the system. You might see this message on the Fulfillment Source section of the Push To Store page.
"We don't currently have any suppliers for this product, but fear not! locate a supplier by clicking on the button below and enter that product URL in the + Custom Source button above."
In order to add a custom supplier, you can click on the "Search for Suppliers" button to go to Aliexpress and search for suppliers selling a similar product. Once you locate a similar product, copy the entire URL from your browser and head back to Sell The Trend. click the Custom Source button and paste the URL you just copied into the popup and click ok.
This will create a custom supplier in the system for you to use while fulfilling orders that you get for this product.