I would suggest contacting PSB directly, but you apparently failed to include a link or any info on what replacement tweeter you are asking about.
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You may not be able to find it because it may be a custom design made for PSB. I would do what Bill_k suggests and contact PSB. Sometimes the mfr can be pretty reasonable about supplying replacement drivers and now you know it will match the other speaker AND it conforms to the original design intent.