So, before I lay out your options, you'll need to know the Japanese word for deodorant when searching: デオドラント. This may also be shortened to: デオ. You may even see something with アンダーアーム (underarm).
A few other words to know:
1. The Flying Pig (Costco) - I've mentioned it before (I feel like I should get some kind of commission for all the plugs they get...), but Costco has a few options, including familiar brands like Speed Stick and Secret.
2. Amazon.jp - Mentioned these guys a lot too, but they carry some American brands like Speed Stick, Secret, Old Spice, Biore, etc. They also carry crystal body stones.
3. 456.com - I just recently discovered this site. They have a wide selection of foreign products. (All in Japanese).
4. iHerb.com - If you are into natural deodorant products, or want to avoid potentially harmful chemicals like Aluminum (the active ingredient in antiperspirants), this is a great place to shop.
5. Foreign Buyer's Club - someone was kind enough to remind me of this resource! Though it is quite similar to Costco, they carry a wide variety of imported (and some domestic) goods. The deodorant selection is on the small side (like Costco) but may be a great option for you.
6. Don Quixote - I went to the Don Quixote store in Shizuoka city, and the brands there were similar to what you would find in any of the above options (plus an Adidas spray brand). You can search for a store in your area on their site.
7. Loft - They carry a variety of deodorant and antiperspirants, including local brands and some foreign brands. They also carry some natural types as well. Check out the site for store locations (all over Japan, in big cities).
8. Tokyu Hands - Carries similar brands to Loft and some of that on Amazon. Some natural types as well. Worth a look. Can shop online or in stores around Japan (major cities).
If you forgo the antiperspirant, there are always other alternatives to hide your sweat.
So, contrary to what many may tell you, finding a decent product (in Japan) to combat your B.O. isn't terribly difficult. Of course, you could always just ask your mom to send you some...
For more, see this post about Japanese Deodorant.
Fellow Japan residents, do you have any other ideas as to finding deodorant/antiperspirant in Japan? Tips?