Starting July of 2016, you can now Copy Estimates to Purchase orders in Quickbooks Online Plus
To access this feature, you must first click on the Gear icon (top right of the scree) > Account and Settings > Expenses > Copy estimates to purchase orders.
Secondly, create an estimate and save it by clicking Create (+) > Estimate > Save.
Lastly, click ▼ to the right of Copy to invoice > Copy to purchase order.
Here is my video explaining the process: