I've complained a lot (read: probably twice) about TF2 Outpost's lack of Killstreak Kit support: you can't search for specific ones, looking for a Kit requires you to search for the Kit type and then sift through thousands of trades looking for the little icon that corresponds to the weapon you want it on (not to mention trades are being bumped while you're looking and you have to hope it's a color or effect you want if you finally find one), but yesterday I played some rounds with the admin of a relatively new site called
killstreaks.tf (his weapon had the name of his site and the description was "You're dead anyway, might as well check it out...", very clever)
It's pretty much just another automated trading site but with Killstreak Kits instead of other stuff, all the Kits are a flat price depending on the weapon and you can easily search what they've got in stock (unfortunately it's not much since they just went public in September and don't have a
staggering number of users)
Killstreakified my Righteous Bison thanks to them, 10/10 would sell unneeded Killstreak Kits to if I actually played MvM