I wouldn't have thought that someone could build an entire internet business around replacement styluses (stylii?), but sure enough there's
http://www.styluscentral.com/default.asp
I haven't even received the 350 I ordered yet, but I've already got this site bookmarked for the future. The pen/stylus combos and the stylus-on-a-lanyard items look interesting. Nothing specific for Sony Readers, though.