Do you use the Job Cost module? In the PO do you indicate which SO it is for and in the SO indicate which PO it is for?

You may want to look at generating the PO from the SO if you know you are going to issue the PO for service.  Look at PO Setup / Generate tab - there you can configure settings to allow generation of POs from SOs.  There's also a setting to use the SO number for the PO - which if used, might facilitate the reporting you are looking for.  Regardless, the generation will tie the PO(s) to the SO.  It's actually pretty slick.  Of course, you can manually add the SO number to the PO.