Which check scanners work with Aptexx, and where can I purchase one?

The only check scanner that will work with Aptexx is the Panini Vision X.  You can order the scanner directly from the manufacturer at https://www.panini.com/en/products-batch-scanners/vision-x

Your preferred vendor may carry them, and they can also be found at Amazon: https://www.amazon.com/Panini-Vision-Check-Scanner-Printer/dp/B004499K2M