Adding a purchasing source to your vendor connections allows you to easily create Purchase Orders.
To set up a new purchasing source:
First go to the Vendors area and find the vendor you would like to set up a source for, then click 'Add Source'.
Search for the name of the source you'd like to add, and if that source hasn't yet been added, click "+New Purchasing Source".
Add all of the contact details for that source, and select a default shipping address.
Click 'Save' and you're all set!