There are a number of 1/700 decal sheets with hull numbers around. I know from experience that this one, from Gold Medal Models, works nicely - though there are dozens of subjects on it that you won't need: http://www.goldmm.com/ships/700-1D.htm .
Another option (somewhat more expensive - but not ruinously so) is to buy any of the 1/700 plastic destroyer kits by Midship Models. Every one of them, so far as I know, contains a decal sheet with enough hull numbers (and other subjects) to outfit a fleet. The kits themselves aren't quite up to Tamiya or Trumpeter standards, but they're basically sound. Each also comes with a sprue of weapons that goes far beyond what's required for the specific ship in the box. Excellent additions to the spares box.
As for aftermarket detail sets - again, there are quite a few on the market. The only one with which I have personal experience is the one from Gold Medal: http://www.goldmm.com/ships/gms7w2cd.htm . I've built a couple of models using that sheet; it's excellent. The only major items it doesn't contain are the 20mm Oerlikon anti-aircraft guns. Different modelers have different opinions on how best to handle those on 1/700 scale. The ones in the Tamiya kit aren't bad, but there's just no way that a 20mm gun shield can be reproduced to scale in plastic. Photo-etched ones can be had from several aftermarket companies, including Gold Medal ( http://www.goldmm.com/ships/gmsaoerc.htm ). Some modelers like that approach; others think photo-etched guns look too skinny. A compromise approach is to trim the shields off the plastic parts (either the kit versions or the ones on the aforementioned Midship Models sprue) and replace them with the shields from the photo-etched set.
Hope that helps a little. Good luck.
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