To make a very broad statement, Japanese people like comic books. Comics are mainstream here. People of all ages and genders read them openly much more so than back home where even if you read the artsy stuff you have to use awkward terms like "graphic novel" to differentiate your refined tastes from those of the stereotypical geek who obsesses over how many stripes there are on Captain America's mighty shield. Comic books are available in massive quantities in convenience stores and bookstores all over the place. Then you have your comic superstores like Mandarake. The catch is they're Japanese comics. If you love manga and you can read Japanese, Japan is the place to be!
Unless you want English-language comics. In which case, your best bet is to order from Amazon Japan. Amazon Japan carries everything from BOOM! Studios to Viz and all that's in between. You can also order some out-of-print books from international sources.
I do a lot of business with Amazon Japan. When Fantagraphics puts out their latest Love and Rockets volume from Los Bros Hernandez, I order it from Amazon Japan. When a new League of Extraordinary Gentlemen comes out, that's where I buy it. I've now got a nice collection of Dark Horse archive books like Nexus, Creepy and Eerie, plus some DC Showcase volumes and two of their Jack Kirby's Fourth World Omnibus softcovers, Hate! and a lot of other stuff the names of which escape me. This past weekend I got It's Tokyo, Charlie Brown from KaBOOM! Studios and Tales of the Batman: Gene Colan Volume One from DC. I paid for the latter at a convenience store Tuesday evening and delivered by a letter carrier early Friday morning; I would have gotten it sooner, but the first delivery attempt failed because I wasn't at home. I also order English-language editions of the homegrown stuff, like Lady Snowblood, Wandering Son and Nana.
You can even buy a very few mainstream monthly titles-- Superman, Batman, Star Wars and Archie. They're not cheap, but neither is a trip to Tokyo. Even in the States, new monthlies are ridiculously overpriced, but here in Japan, they're even more so. You have to be really desperate for a fix to pay these import prices. If you're willing to wait, it's much more economical to wait for the inevitable paperback collection. If the exchange rates break your way, you can pick up some bargains via Amazon.
You don't have to have a credit card, either. You can use one, of course, but as I wrote, you can pay at a convenience store-- the process involves using a digital kiosk (it also allows you to buy concert and airline tickets plus pay your bills), entering a few numbers, printing out a little slip and having the cashier handle the rest. My favorite option is COD, or cash-on-delivery. You get your books faster this way. The delivery person shows up, you give them the money and everyone is happy.
Especially you, because you now have something fun to read.