Address Book field for the Customer account number for supplier

We have suppliers such as Staples where we have an account number assigned by Staples to use when we send orders.  We have multiple accounts at Staples based on which location is placing the order.

Where is the best place to put this account number in the address book.  It looks like the best place for this would be the Long Address Number.

Currently AP users enter the account number in the remark field when entering invoices, so the number will print on the check stub.

Looking for ideas to address this.