How to default Requisitions source from internal inventory across organization

Hi There,

We have Oracle outof the box iProcurement Requisitions responsibility and we stock items internally as well as we source from external suppliers.

We wanted to have the default of all the requisitions raised to follow internal inventory first and only specific users (with customized responsibility) to do external requisitions.

Sourcing rules for all items have suppliers setup.

Please advise.

Thanks

Venkat Desu