Just buy a new horn driver - if you go to parts express you can get one for fairly cheap.
Or you could go the really cheapo route.
I've made one using a 4" speaker. Make a little box to put the speaker in, and then glue a funnel face-down over it with a hot glue gun, and attach the rubber tubing to the small end of the funnel. You can get a bundle of different sized funnels at Walmart for like... $2 in the section where they sell Kitchen utensils.
You don't need much power to drive the speaker.
One of my friends made one this way, and had the whole thing encased in a box, with just the small end of the funnel sticking out through a hole so he could attach the rubber tubing to it.
The thing works great.