Finding an existing purchase order in QuickBooks Online is pretty easy and I'd be glad to show you how, acequip.

The Advanced Search feature in QuickBooks Online allows you to look for an existing purchase order. Let me walk you through the process.

1. Click the Magnifying glass icon at the top and hit Advanced Search.
   
2. Select Purchase Orders and click Search.
3. Choose a date from the Date Filter.
4. Hit Apply.
   
5. Find the existing purchase order from there.
   

You can also go to the Expenses tab and find the existing purchase order from there. Here's how:

1. Go to the Expenses tab.
2. Click Filter.
3. Choose Purchase order and filter the Dates.
4. Hit Apply.
   

I'm leaving you this article to learn more about searching a transaction in QuickBooks: Search for transactions. It also gives you steps on how to search by customer and vendor names. Additionally, the following articles are good references. These will provide you steps on how to process a purchase order from an estimate as well as steps on how to track and run purchase order reports in QuickBooks Online:

Create purchase orders in QuickBooks Online

Copy an estimate to a purchase order

This should keep you moving today, acequip. Don't hesitate to let me know if I can be of additional assistance while working in QuickBooks or need more help with finding your existing purchase orders. I'm always here to keep helping. Have a good one!