That is a very rare size of tire, not manufactured anymore except in very limited runs by Yokohama and Pirelli for the RSR/Pantera/Lambo/Cobra crowd.
Amazingly, there is an ad in our PCA-SDR parts classifieds for some used ones:
"For Sale One Pirelli Assimetrico 345/35 ZR 15 P Zero Virtually new $100 and a P7R 345/35 VR 15 in good condition $75.
Contact - E-mail Brian or Tel: 619-238-1977." Those ads get very stale, though, and I would not count on it being a current offering.
The Tire Rack has had some stock in the past, but I don't think currently. Give them a call, though, not everything is on their website. You can occasionally find someone who has stashed some. I would try Dave at TRE in Hollywood
http://www.tremotorsports.com/Home.htm
or Ernie's Alloys in Miami, FL (advertizes in Pano)
or Super Buy Tires in Bellflower:
http://www.superbuytires.com/tires/345/35R15
Good luck,
TT