Do like this: check out my screenshot :P
I was going to write how to search for killstreak weapons, but i'm too lazy to form normal sounding sentences that makes sense, so here it is plain view: http://imgur.com/KaadKi0

The top left yellow text = WEAPON = select which weapon you want to have the killstreak version of
The lower left text = WRITE KILLSTREAK HERE = Simply write the word Killstreak in that text box, then click on the "Search!" button.

Ta-da!