draconis393 (weapon_icons) creates 'em, and I convert and post 'em. Please credit her, as I only converted them to a format usable in GIMP.
Original PS brushes can be found here
at her Livejournal or at her deviantart.
To install, unzip files into the brushes folder in your GIMP program files (usually C:\Program Files\GIMP-2.0\share\gimp\2.0\brushes in Windows or /home/username/GIMP-2.0/share/gimp/2.0/brushes in Linux) and then restart GIMP, or refresh your brush dialog.