Pedro (user ppbon on here) has installed S calipers up front, you can read about it on his website here:
Well, now I can't seem to find the link. The website is here:
http://www.pedrosgarage.com and I think there is an article in there somewhere. There are lots of other good articles so even if you don't find the brakes one it should be a fun visit to the site
Centric sells the dust boots and inner seals to rebuild our calipers.
For a 98 boxster these would be the part numbers (if this link works)
ELECTRONIC CATALOG - LOOKUP BY APPLICATION
Front: 143.37013
Rear 143.37009
Centric also makes the brake hardware kits (the clip, rod, and spring thingy) if you want to replace those.
Steve