I'm looking for a christening outfit too so can't help I'm afraid. Not helped by the fact I'll be 16 wks pg with dc2 and no idea how big bump will be. Monsoon have some nice ones in, but I appear to be between sizes ATM. Same with phase eight, who have a sale on if that helps.
As a fairly regular churchgoer I would say don't go overboard. Whenever there is a christening at church the christening party are dressed far more smartly than the congregation. I would just buy something you can wear again.