You can purchase money orders at post offices or Western Union, etc.
You should NEVER, I reapeat, NEVER, send cash to anyone! You will never get your money back and the person you are sending the money to will not receive the money. Once you purchased the money order, you should send it by priority mail to ensure safe delivery.
If you purchased something through an auction site (for example: ebay), you can also pay with www.Bidpay.com. This is a western union website, you can purchase the money order online and they will send the money order to the seller so you don't have to go buy and mail the money order. It's really convinient but they do charge you a pretty high shipping charge.