I disagree with those above that don't think it is a worthwhile mod - there are many advantages rigid has over suspension. If the bike is never ridden on rough trails then I would certainly prefer a rigid fork. I don't know of a good one to recommend - one of my bikes has a surly instigator fork and it is a pig.

EDIT: I have seen carbon mtb forks for sale online for not too much dough. I would expect a cheap carbon fork to be pretty tough for city riding. A lightweight steel fork would not be my first choice but would probably be ok too.