Morel MDT 12 are like $70 a pair and are a decent little starter tweet. I've got mine crossed at 2500Hz. They're a quite good little tweet to start out with and will do exactly what you need, until you decide to drop some decent coin on a high end set.

<edit> Using a passive from another set complicates things a lot. The Morels are 8-ohm, so that may be a dealbreaker. Damn.