Hi marc-harmonique-.
Creating sales orders is a great way to track services and items you order for your customers so you can have them all in one place. I'd be happy to help point you in the right direction with this.
If you wish to find a third party app that can help you create Sales orders you can go to our third party apps page directly from your QuickBooks Online account. Just click on the Apps button on the left hand side and search for what you are looking for.
Another option would be to use QuickBooks Online Estimate feature which is similar to sales orders as neither are posted to any account until they're created as an invoice. You can create an Estimate in QuickBooks Online by following the steps in this tutorial.
If you have any questions let us know and we'd be happy to help.