Re: Velocity Locking Screw
You can get replacements at a hardware store. I replaced mine with one that has a knob on it so I can adjust it by hand. That being said, I find them pretty useless. The original one worked loose all the time. And I never found a field that cared if I had the screw or not. My guess is that tournaments might, though.
One thing I found did help was to put a small piece of rubber between the screw and the marker body, so that when you tighten the screw it doesn't shake loose so easily.