One of the holsters came from an Ultimate Nick Fury, the one you can't see very well in this picture. The other was just one that I found that faced the opposite way, I just sculpted over the top of it to make it look like the other one, this is the one you can see better in the picture.
I'd put some aluminum foil, plastic, etc. around the head of your figure (to protect it from your modeling compound adhering to it), and then sculpt the hood around that. You can also pad out areas on the head to give the hood's shape more volume.
The way I sculpted the hood for this guy was pretty easy. But only if you happen to have a 6 inch Big Chill figure from Ben 10 Alien Force. The figure comes with a hood that I cut in half and used as a mold. I just pushed some sculpt into the hood, let it sit for a bit to harden up, then took it out and put it on the figure. I had to leave the bottom open and add that as a separate piece after attaching the large part of the hood. Check out you fodder (or other random things around your house) that could be used to get the basic shape. Just take them out before your sculpt bonds to the mold permanently and so it can be adjusted a little if need be.