Hey bro, I'm actually enlisting in the corps soon. Ironic huh lol. So I'm assuming you have a good weight training routine? It's gonna be hard to bulk up while doing all that cardio and crazy shit you guys do. I'll try to throw in some good advice though...
First off is the protein. The most basic supplement. You can get some good protein here;
Whey To Build Muscle.
If you want to get a little extra boost, then grab some GEAR pills from the same site.
Another great supplement that will help you a lot and is very cost effective is creatine. You can get some creatine monohydrate for cheap at that same site. This will put on some water weight quickly, but it will also help you increase muscle endurance.
These supps in my opinion are almost necessary. They will help you a ton just by themselves. Also look into some kind of slow absorbing protein before bed. When you sleep you burn a pound every night. This will help reverse that.
If you wanted to get even more advanced, you could try a pre-workout supp like n2kts (again, from that same site). It's an awesome product and it'll get you pumped for the gym every day.
Really the simplest way to bulk up is to eat more man. What's your training routine look like? Oh and like axismundi said a good multivitamin
Just curious, but what's your MOS?