Are you going to be there for a significant amount of time (at the very least 3 months)? If not, I wouldn't bother.
To answer part of your question, yes, you can, indeed, import AAS for personal use and this is perfectly legal. Don't expect to just be able to walk into a pharmacy and pick out what you want, though. This isn't Thailand.
I will not give you a source since you only have one post to your name, but you should be able to find places to buy whatever you need by putting the proper search terms into google. It helps if you can read Japanese or at least have a Japanese friend, but it isn't completely necessary, if you're really determined.