Tracking Expenses for State Sales and Use Tax Audits

I am looking for a way to track expenses for offices located in multiple states for sales and use tax purposes, especially when it comes time for an audit.  Currently, we do this by department code, but we are also looking at consolidatiing department codes. When pulling data for an audit we typically run detail on the departments associated with the state the audit is being performed on.  Some invoices the expense is processed in AP and some are posted directly to ledger. This being said any suggestions on how to track expenses other than by department code to capture expenses for a specific time period or in this case audit period?  Thank you!