Queries - Unique reference [Order Ref]

I have table which contains products that I wish to order from different suppliers:

Supplier A --- Product X --- Qty 200
Supplier A --- Product Y --- Qty 100
Supplier B --- Product Z --- Qty 300
etc..

I want to raise a document that will list the products I wish to order from the supplier using the above table and give the document a unique reference [Order Ref] that I can use to send/mail my order to my supplier.

I will then have:-
Supplier A --- Product X --- Qty 200 --- Order Ref 123
Supplier A --- Product Y --- Qty 100 --- Order Ref 123
Supplier B --- Product Z --- Qty 300 --- Order Ref 124
etc.

I think what I need to do is to make a tempory table to hold the above and then append to my existing order table.

I just do not know how to generate the unique order reference, and update the field in my table with the unique order reference.