Thanks for posting in the Community! It's important that you are able to find the cheques that you need. QuickBooks Online is great to have, it gives you the option to search for amounts that may not be easy to see. I can proved some information on how to find these amounts.
In QuickBooks online, at the top of the page you will see a few icons. There is the Help icon, the Magnifying glass, the Bell and the Gear icon. If you click on the Magnifying glass icon and enter the exact amount of the cheque you're looking for, it will pop up in the list under the number entered. If it doesn't pop up here, this would mean that it has not been recorded in QuickBooks.
If you have any other questions, feel free to reach out here.