How would I find a customer invoice by the number?

Have you tried going to the Edit menu and selecting Find? 

Thanks for reaching out to the Community about this, youngsweld.
 

You can find a customer's invoice by it's number by accessing your Invoices screen and sorting them by NO..
 

Here's how:

1. In the left navigation bar, go to Sales, then Invoices.
2. Click NO. to change your list to where it's sorted by invoice numbers.
3. If necessary, select NO. again to change it from ascending (▲)/descending (▼) order.
4. Locate the number you're looking for. Each one will be listed in your NO. column.

You'll additionally be able to find many detailed resources about working with QuickBooks in our help article archives.
 

If there's any questions, I'm just a post away. Have a lovely Thursday!