That's a great idea for a gift! I have Mikimotos and I love them. DH gave them to me when I graduated college (DBF then). But, we do have a friend who owns a family jewelry business, and he said that with Mikimotos you are paying for the name - that they are great quality, but you can get equally great quality for less than the cost of Mikimotos. Anyhow, if Mikimotos are expensive, and can be more expensive at WDW, and you don't have to pay tax in NH, you may be better off shopping around at home and bringing them with you to WDW. And, I think you will be under less presure if you shop around at home, rather than having to pick them out on the spot from a possibly limited and possibly over-priced selection.
If you buy them in advance, you can give them to her in some quite or romantic spot in Japan or Epcot or WDW. I think that could be just as nice as actually buying them there. Just a thought....It's like the engagement ring -- doesn't matter where it's bought, but more where it's presented that holds the memory! (DH bought my ring in Detroit, but proposed in Salzburg -- I certainly think more about Salzburg when I look at the ring than I do about Detroit! LOL) If you give them to her in WDW, that's what she'll remember, regardless of where you bought them.
Whatever you decide, enjoy your trip and congrats ont he anniversary...I'm sure you wife will be very thrilled with your gift idea!