In TF2, if I wanted to get a custom weapon with special qualities (such as Valve quality), how would I do it? (See Karma Charger's videos on youtube concerning weapon demonstrations)
In order to give yourself weapons of irregular/unobtainable (through trade) qualities, you would need to use server plugins. If you're not familiar with plugins, they are essentially add-ons to a server (notable examples of popular plugins being !RTD and !ROBOT).
You can give yourself weapons of these qualities using the SourceMod [TF2Items] Give Weapon plugin. Keep in mind this is a server plugin, meaning that simply dragging the files into your TF2 folder would be wasting your time - research on how to create your own dedicated server (Windows or Linux) if you're interested in recreating some of what Karma Charger does.