If you really MUST go with a money order, try 7-11 or something. If they ask, tell them you're getting it for your mom to pay the light bill or something. 7-11 and other convience stores are usually less nosey than USPS.

If you can, and if you have a bank account, just go and see if they sell gift Visas. They're like prepaid credit cards and work just like a regular credit card.