Seth asks where he can find aftermarket chainguards for his Bianchi Castro Valley bicycle. I went on a hunt for plastic chainguards about three years ago and was unsuccessful. Last year, somebody asked where to find bicycle chainguards at the Bicycle Commute Tips blog and received no answer.
I've quickly searched the USA bike distributors catalogs for chainguards. SBS carries replacement chainguards for theirk Torker and Redline bikes, but I didn't see anything aftermarket. J&B looks like they carry a number of plastic chainguards. Most are for the brands of bikes they have (e.g. Sun & Cykel), but I see a number of generic items like "Chainguard 20" no clips." J&B does not sell to the general public so you'll need to go to your local bike shop and have them order the chainguards from J&B.