Or, if you wanted to be a bit more polite, you could ask some staff member and show them your shopping list. Worked great for me 
But if you want to find them yourself, Naphthalene once wrote up a nice explanation on how to find Manga. For the other books, they might be in various places but I was rather successful in the „Japanese Authors“ section (there usually is a huge sign above the shelves) and then go by (Japanese) alphabet and maybe that is still subdivided for famous authors.
If you cannot find the books at Bookoff you can still buy them new in Japan and it might still be cheaper 
Edit: The post I mentioned