The voltage in Japan is 100V instead of our 110V, but I'm almost certain the DS plug is the same for either version. The voltage difference is so minimal, almost everything electrical works just fine in either country.
If you're going to be in Tokyo, Akihabara is the part of town that is known for its electronics. It's like a Fry's Electronics that encompasses multiple city blocks.
I have read that running Japanese equipment on 110V will wear it out faster than if it were run on 100V. For my Japanese PS2, I bought a converter that adjusts 110V to 100V. I plug the PS2 into the converter and then the converter into an outlet.
However, you are merely charging a Japanese DS Lite on 110V, so I'm guessing that it wouldn't be a big deal at all. And low and behold, I just took at look at the AC Adapter for my US DS Lite. Interestingly enough, it's a 100V adapter designed for both Japan and the US. So the final answer appears to be that there is no difference at all between a Japanese DS Lite and a US DS Lite; even the adapters are the same. You'd just switch the menus to English and off you go.
I was in Japan in July, and DS Lites were nowhere to be found. I was in about four dozen electronics stores and never saw one for sale. List price was about 17,000 yen, which is about $140. I just checked availability now on Amazon Japan. No DSs are available directly from Amazon, but the jet black ones are on-sale third party starting at 21,000 yen (about $180).
Have a great trip, though!