Its unlikely you'll find a database. I presume you've searched - as many people post little bits that may help you.
But if you have say 20 of an item and use one, then you just need some code to reduce the total by one and check if its reached its re-order amount.
You could put the code in the OnClick event on the "confirm order" button because up till then the order could still be changed.
edit - try the Northwind database, that has orders and stuff in it
Col