Is the Infinity tweeter a ribbon of some sort? If so, Parts Express carries a variety of ribbon and planar tweeters. Of course, if you wanted to be *really* anal, you could just choose a highly linear/low distortion planar tweeter, such as one made by BG, measure your Infinity speakers, then just use a good parametric equalizer to achieve a similar response characteristic (at least in terms of frequency response, distortion is another matter). However, that would take some time to set up the measurements and such. Still, I think it would be the optimal choice.
As for good sound, JL Audio makes good drivers. They are often overpriced, in my opinion, but they tend to be very linear and of high quality.
You should also be aware that you will need to do something to compensate for the magnetic field radiated by your JL Audio drivers. They do not appear to be shielded. If you plan on placing your center near a CRT TV, they will need a shielding can, bucking magnet, or some type of electromagnetic shielding that will prevent the field from straying outside of the speaker cabinet.