Bill, I don't honestly know which ones they have, but
http://www.tdi-plc.com/IMPREZA.html carry the HKS plugs... email them and see. These folks....
New Jersey
(Sarizer) Rally Motorsports Hackensack 201 / 968-0800
New York
Paisley Automotive Hempstead 516 / 481-2815
and
Rampage Racing Woodside 718 / 396-1300
carry the HKS products, you could try calling them- Brian