For multilocation quotes always build them by creating the Opportunity from the top-level hierarchy account record. All location account records at or beneath that level will then be available for quoting.
During the quote process, you will have and use only one Opportunity throughout the quoting process even though you may have many locations, multiple Suppliers, and numerous products.
To add locations oress the "Add Locations to Include in Quotes" button below on the right. If this Account has only a single location it will be shown. If this is a multi-location account, this location and all other locations below this location in the Hierarchy will be available for selection. You may select up to 1000 locations per quote request.
For information on how to use Account Hierarchies click here.
You'll see all the Accounts in the hierarchy. In a view something like this:
Press the Add locations button to add those selected.