To find an invoice by sales order number is not supported in Sage 50 Canadian.  

The consensus seems to be that the best work-around is to put customer order numbers into the 'ship to' area on the invoice.  They will be searchable with a general text search.  

You could add your voice to existing requests under Help | Feedback.  Displaying PO and SO numbers in reports and making them searchable has been requested hundreds of times over the past decade.  It seems to be something they want to keep out of the Sage 50 Canadian software.

To find existing invoices' Sales Order information that had been entered in the Sales Order field, you would need to use a 3rd party reporting tool designed for Sage 50, such as XLGL, Sage 'Intelligence', or study the table structure and use ODBC with Access, LibreOffice, Crystal Reports, or some other such tool.  There are Sage 50 consultants who build reports and can assist you with implementing a solution.

Item transaction history for a particular part is available under Reports | Inventory & Services | Transaction.  (Select to show 'Sales' only.)