Sales: Assign items to specific customers

[Spirits]

We run a business with multiple items sold (approx 300 different items) and have many customers.  It would be very efficient if we could assign particular items sold to each customer as say a drop-down list rather than choosing an item from a very large list. At the moment, this information, by customer, is maintained in a spreadsheet as each customer has specific items sold to them directly.  Of course, if a new item is sold to a customer, than the larger items list is still available for selection.  Thanks in advance if someone can assist. 

"Assign specific items sold to individual customers"