You can order a product from a supplier other than your default supplier without changing the default setting. Additionally, you can set a default supplier for an item on a proposal, such that when an order is created from the proposal, the order will default to the chosen one-off (not catalog default) supplier.

To change the default supplier on an item from a proposal, find the product line item in the area and click the line item menu (on the right), then select "View/Edit Supplier":

Select the preferred supplier by clicking "Use this supplier instead" below the supplier name on the left. You'll then have the option to make it the default supplier for that item in your catalog, or to leave that the default supplier only on this proposal:

To create an order for a product using a different supplier than your default, go into the catalog and select the item you want to order. Once you're on the product details page, select More Suppliers.
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Next, select "Add to Order" for the Supplier you'd like to order from.
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If your supplier is missing, scroll to the bottom and select Add a Supplier. You'll then be prompted to choose the new supplier.
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Locate the supplier you'd like to add to the part, manually enter your dealer cost, and then the option to add the item to orders will appear.