For most electronic stuff, I have found JGtech to have pretty good prices (www.jgtech.com). I haven't checked for the items that you are interested in though.
Why not just do a quick Google? Marine SuperStore aren't the cheapest. The Sea-Me is probably cheapest at JG Technologies, but they won't bother selling Nasa AIS stuff. If you really want a Nasa AIS engine, try AllGadgets - but you'd be better off getting a proper dual-channel receiver such as EasyAIS or Comar.
One thing to check is shipping costs, sometimes they make-up the differences in prices. personally, I have used Marine Electronic Services Ltd., happily, but I'm not sure where the cheapest is for your products.
Try http://stores.ebay.co.uk/Online-Marine-Mart for NASA I've just had brilliant service and value. The lady who runs it is Angie Mulley. Is it coincidence that NASA's MD is Alan Mulley?
Mike