My dad has just had this done for his brother's 60th birthday. He went to a local company who mount and frame pictures and they were very helpful. They made a shallow frame for the medals but would've made it in the style of a box-frame if he'd have wanted it. I'm not sure how much it cost but I know it wasn't particularly expensive.
As helliebobs says any picture framer will be glad to do this for you.
There are lots of them about and surprisingly inexpensive.
Support your local picture framer I say!
The results are always better than buying off the shelf stuff and this is the sort of thing they do everyday so you can be sure it will present your dads medals in the manner they deserve.