well whatever you do don't go waste your money on some sfi orthodynamic tweeters, they're not that good especially at $30 including shipping. i would say just canabalize some cheap koss headphones, i have had good luck with these and they are about one step down from my all time favorite porta-pro's.
http://www.amazon.com/Koss-Folding-Portable-Headphones-Control/dp/B0018ME6YU
I could enter the whole debate on orthodynamics but i don't want to get into any arguments. basically, in my experience, with an orthodynmaic speaker that has both front and rear magnets perforated, they still don't sound as good as the speakers i canabalized from the headphones i listed above in open air. And perforating the rear magnet makes the speakers sound a ton better.