They often appear on EBay, if you know how to buy stuff there. I did a quick search and didn't see anything right now, but I have seen them there often.
You might also email Cal Lloyd at the Armed Guard Veterans Assoc (firstname.lastname@example.org). He might know of another source.
Thee were no special medals or ribbons specifically for the Armed Guard in WWII, so to figure out what medals he is entitled to would take some effort. You'd need to know what ships and/or stations he was assigned to and the dates for each. From that, I can tell you which medals he is entitled to, by referencing the Navy Awards Manual (which I am currently transcribing so it can be posted on the Internet for everyone to access - a massive project!).
Good luck. I did a similar display for my Father. Half the fun is tracking down the information on what awards he was entitled to.