Dario,
I'm assuming you'll be travelling for quite a while if you want to develop as you go - assuming too that you'll be bringing whatever tanks, reels, and other items needed for developing. If you want to go to a real store that has chemicals, then your best bet will be Yodobashi Camera (sometimes Bic Camera too - their selection is much smaller, but sometimes they carry film or chemicals that are absent from the shelves at Yodobashi). Most large cities have one (although Tokyo has multiple) and I'm pretty sure they all have a darkroom section. That said, not every store is equal - some are better than others in terms of stock. Here in Kyoto the film, paper, and developer options at Yodobashi seem to be getting fewer and fewer every time I go in. Personally, I can order from Yodobashi and have it delivered to my home (for free) within 24 hours, but if I try to do it through the store it will take 3-7 days. Here's a look at their
darkroom chemical selection (Google automatically translates the pages for me, but even if you don't do it, you can tell by the pictures and the prices what you're getting). They do not sell Rodinal though - if you want that then you definitely have to go through Silversalt, as Ben has already mentioned. The guy who runs the site understands English, so you can email him if you have any questions or concerns - there's no storefront though, so you'd have to get whatever you want delivered to whatever accommodation you're staying at.