You cannot buy a physical USPS postal money order online. You must purchase them in person at a Post Office location using cash or a debit card. How to Get a Money Order
: This is your only proof of purchase and is required to track or replace a lost order. Online Alternatives buy postal money order online
Since you can't buy them online, here is the standard process for getting one at the Post Office: You cannot buy a physical USPS postal money order online
If you need to send money digitally without visiting a physical location, consider these online-only services: Sending Money Orders - USPS buy postal money order online