You can't get one at any point today? If you can't get one today, show up tomorrow, go upstairs and put in your paperwork. Then tell them you forgot to get the money order and you should be able to run out to the nearest post office to get one. The wait for medical is long so you should have time to dash out and come back. Or, you could wait for medical to be done and if you pass it, they'll send you for fingerprints. You could tell the fingerprint folks that you don't have the money order and they'll tell you to go get one if there is still time left in the day. I'd get one today even if I had to ask someone to go get one for me and pay them back.