Sailor Moon in Japan

Yes! I asked the same question when I went to Japan in 2009. I'm hoping your stop is in Tokyo.

If you mean Anime DVDs, you can probably find them at any Tokyo (or any Japan) media/DVD outlet you find... however you'll only find them in Japanese. If you're looking for English dubs you'll be out of luck. Those are extremely hard to find!

If you mean toys, t-shirts, and the like, I would try going to some toy stores. Kiddyland in Harajuku http://www.gojapango.com/tokyo/tokyo.php?poi_id=1417 - It's a 5 story toy megastore with TONS of stuff. I was specifically on the lookout for Sailor Moon stuff too (I traveled all around Japan and strangely did not see as much Sailor Moon as I would have thought) - and I found some great stuff at Kiddyland, so I would start there.

Other toy stores in Tokyo: Yamashiroya Toys in Ueno, Hakuhinkan Toy Park in the Ginza.

Also get yourself to Akihabara and wander around, you will find a lot of stores with toys and electronics, they are mainly the bigger stores. Lastly, check out Ameyoko, a busy market street that might have anime stuff - http://www.japan-guide.com/e/e3012.html - it's worth checking out even if you aren't looking for anything in particular.

Hope this helps!