I designed them and had them made my Tarantula MFG
I believe he made extras.
"Universal mini volumizers:
These were made a while ago mainly for style, but offer a volumizing effect due to the small chamber machined into it. They are available in two sizes, small and large. The end is threaded 1/8-27 NPT, which is the standard tapered plug thread common on most markers. These are available in raw aluminum, but I have a couple in gloss black anodize. $10 each. Email for more info."