Are you using Sales Commitments? And do you want to look at posted Sales Order, historical Purchase Orders? Or unposted Sales Orders, and closed (but not historical) Purchase Orders?
In general, if you are using Sales Commitments these would be the tables you want to take a look at:
SOP10100 (Sales work header)
SOP10200 (Sales work line items)
SOP30200 (Sales history header)
SOP30300 (Sales history line)
SOP60100, SOP to POP Link table (to link the sales order line item to the purchase order)
POP10100 (Purchase order work header)
POP10110 (Purchase order work line)
POP30100 (Purchase order history header)
POP30110 (Purchase order history line)
Also, if you want to see the fields in these tables, as well as possible ways to relate them, you can use Microsoft Dynamics GP>>Tools>>Resource Descriptions>>Tables to view more information.
Take care,
Christina