Perhaps if you have no time to go to the stores, you can just come up with something quick like I did before. Just take any old belt, and slide it under your arm, voila. You can decide to attach whatever you want on it with some black electrician tape. The easiest and most comonly found are old batteries, especially the big ones, that you can use for the bullets, or a flashlight too to add more realism.
OR:
You can sacrifice an old sportsbag that you dont use anymore, and cut the strap off and sow it, or gluegun tape it together.
Use a green, black or grey shirt and wear it bandana style.
BTW:
If you own already the Gi joe footlocker (dvd episodes boxset) dont forget to use the dogtag in it, it'll look great with your costume.
