To be able to select all accounts from the CRM when creating a purchase order, you must have a certain setting correct in the CRM's account sheet.
In the submenu Accounts & contacts of the CRM module go to the tab Details under Accounts. Now you can check in the account sheet of the certain account that this account is a supplier. This also applies to all other accounts.
Now when creating a purchase order, you'll be able to select all these accounts from the CRM as suppliers. If needed, you can also create a new supplier here.