porsche monobloc calipers are made by brembo and are identical fitment, so with the right boxster calipers, you will be able to fit them onto ICR 305mm discs and the oe mounts, although from what i see the plumbing is different. i have had porsche 996 calipers bolted up onto the same mounts as my brembos. rim clearance is actually worse than the brembos, by the way.
try a search for trainers scrap yard, dont know if they do mail order, but they should have loads of porsche calipers. also try bira.org for details of mounting porsche calipers, all details are on 5 stud applications though. has anyone redrilled 312mm tt discs to 4 stud?????