Maybe you couldn't back then, but you can now buy them on line at:
http://www.usmedals.com/medals.html?s=navy&p=vietnam
This site has medals from all of the services and wars. It doesn't look like you can buy a Medal of Honor though - it's probably against the law.
You might be able to buy the medals all day long, but you won't have the Certificate and orders that say you were awarded the medals.
OR you can do like some people and just photocopy and put your name on it.