On the cheap end, there are always the $0.20 onkyo 4 ohm tweeters at partsexpress (you may need to extricate them from their mounts).
Likewise, apexjr.com has a bunch of $1 tweeters .
These aren't ribbons, mind you, but should work well at about 5KHz upwards. Not much power needed if you array enough of them (for normal listening).
If you wait for a month or two, I can give you my first hand impressions on these ![](http://www.audioroundtable.com/emoticons/smiley.gif)