Shopify allows to add only one supplier for each product but in some cases you can order the same product from multiple vendors.
For this reason you have the ability to add alternate suppliers to make it easier to filter product by vendor. Here are steps to add an alternate supplier to a product:
Open Oder product view
Click Edit suppliers button
Enter supplier or select existing one from list
Click Save
One the Products page you will be able to see the alternate vendors right below the name of the product. To filter products by said supplier, just click on the tag and it will be automatically used as search term.