Also the fact that most Japanese wont want to hear your 'potty mouth' until you are at a certain level of friendliness with them. If you plan on going to Japan, do so, and wait until you have some good friends, and you will find that you pick it up very easily.
Who are you planning on saying those things to anyway?
I have had Japanese friends for years, know lots of 'those' words, but still dont feel comfortable with putting them into everyday conversation with them. Believe me, what you might take as 'funny ha-ha' they could get offended with.
However if you are just wanting to learn it to have a bit of a giggle to yourself... eat your heart out. I had a book of slang which I thought was really funny. Showed it to my japanese friends and they nearly had a heart attack at some of the stuff that was in there. Even my slightly crazy friends had some warnings about some of the phrases.
You need to immerse yourself in everyday japanese and then you can slowly pick out what is ok to say and what isnt.
Probably you have some friends that can teach you the bad stuff that wont recieve a bad reaction? Ask them, dont waste money on books when you honestly wont need THAT many.