There's a newer version of this post here: https://odootricks.tips/about/use-server-actions-contact-customer-supplier/
In Odoo 13, the fields "Is a Customer" and "Is a Supplier" on a contact have been removed.
Instead there are two new fields for Customer Rank and Supplier Rank but they are not displayed, and so cannot be changed.
If you create a contact from the Customer menu option it will have Customer Rank set to 1, and can then be selected as a customer on a Quotation / Sales Order. Similar logic applies for suppliers / vendors.
If you want to have a contact who is both a customer and a supplier, you can create a Server Action to set Customer Rank (or Supplier Rank, or both, to 1.
You need to be in debug mode. Then navigate to Settings / Technical / Server Actions
Click on "Create"
Now enter an action name ("Set as Customer"), model (Contact / res.partner), field (Customer Rank), and Value (1) and then press "SAVE"
The next step is to create a Contextual Action by clicking on "CREATE CONTEXTUAL ACTION"
And this will be available from the "Action" dropdown:
It's simple to make this server action update a contact to be both a customer and supplier:
