Due to a recent bug in the Gun Mettle update (which has since been patched), Specialized Killstreak and Professional Killstreak kits/fabricators can be found in MvM and applied to any item (including hats).
Since this patch, the only way to obtain such items is via the Steam community market, and they're very expensive.
I'm considering buying one, but I need to know beforehand: do the Professional Killstreak effects apply to the hats? e.g. If I have killed 10 people with a Killstreak shotgun and am wearing a Professional killstreak Gibus, will the Professional Killstreaker show (even though I have no Professional Killstreak weapons)?