This is all just guessing as the picture is kind of small to see the belt, but...
I can't tell for certain, but that buckle kind of looks like a side-release buckle with a metallic finish (like this
). As for the belt itself, perhaps you could use some black webbing? The wider portions could be made with black material of some sort with added belt loops (at least it looks like there are belt loops to me). And the pouch on the side, it seems like you could find a large black pouch fairly easily at a military surplus store and then just glue or paint on a red cross. I've found some similar black pouches at a local military surplus store myself.