Other than just visiting their website, contact
Injen [
injen.com] directly:
TEL 909.839.0706 / FAX 909.839.0427 / 244 PIONEER PLACE, POMONA CA 91768
Although Injen.com does not have an online web store, they probably could locate exactly the Injen parts you're looking for.
Also, the "2000, 2001, 2002, 2003 Celica GTS L4 1.8L Billet Aluminum Spark Plug Wire Cover" is still listed on their site:
http://www.injen.com/search.asp?stop=1&Year=2000&MakeID=13&ModelID=24