Have you ever designed and built a speaker before? It's really not as simple as selecting two drivers and sticking them in a box with a random x-over. In order for it to sound decent, you will need to select both drivers based on their actual parameters and tailor a crossover to those parameters. Without any measurement equipment, you are basically just guessing.
Vifa makes great tweeters in nearly every price category and that would be a good place to start. Seas, Audax and Scan-Speak also make great tweeters. You can check the forums at PE - there are some serious DIY guys there.
GR Research has an excellent soft dome tweeter for $60/pr., but I would recommend you actually take a look at the A/V-1 kit. It comes completely unassembled, but has all the parts you need except the cabinets. The owner is a great guy and is usually pretty good about responding to e-mail questions. Customer service is excellent, as are the speakers.