To see what suppliers carry a product you aren't authorized for, find the product in the catalog.

Once you click on the specific item, you'll be taken to the product details page. Here, you'll select More Suppliers to view all vendors that carry the item.
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From the list of suppliers that offer this specific product, scroll past any existing suppliers you're already setup with, and click "I Buy From This Supplier" for the supplier you'd like to setup for this item.
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