In order to sell merch, you need a storefront and a willing customer base. I would recommend setting yourself up with a tiktok/youtube/instagram channel (come up with a cool name to brand yourself with) and start publishing short videos of your rides and experiences in Japan. You can try to develop a fan base, who might be interested in merch. Especially if you are at an event or something where you are speaking or tabling, that's a good place to sell merchandise.
Wataru Iino is a good example of someone who has made a brand of his adventuring, more catered to a Japanese audience. I'm sure he's on other platforms but here's his youtube:
https://www.youtube.com/@wataruiinosmilerunningchan952
Unless you have a place where you can store t-shirts and the ability to fulfill orders, or have an agreement with shops to carry your t-shirts (either by consignment or buying them wholesale from you), I would warn against sinking money into printing shirts. You can try on-demand print services like
RedBubble that will produce and ship your product with your design (which is very little overhead for you and you can sell worldwide), but you won't be making a whole lot per unit sold, which means you'll need to sell a
lot of shirts for it to be more than a few bucks here and there. Also, these kinds of items are more impulse purchases, which is why I think you'll have better luck tabling at events or wholesaling them to tourist stores, or promoting them through your own sales channels. But then you have all the overhead and up-front costs, plus you have to deal with a bunch of t-shirts that may not sell as fast as you'd like.
Now that I know what you're asking for, I would actually have a design that says "Adventure Cyclist" and "Japan Tour," with a cool image sandwiched in the middle, like a Samurai on a bike, or a loaded touring bike on a picturesque Japanese byway, or even just leaned against a conbini in the countryside. That's the kind of thing that may appeal to people who see themselves as Adventure Cyclists, coming to Japan for their own tour. Smaller merch items like stickers, enamel pins, etc might also be worth looking into. Sites like RedBubble will let you upload a design and then choose which items you want to offer with your image.
Selling merch is one of many ways to capitalize on your adventures, but it's not passive by any means. If you build up an audience online, you can have exclusive content for channel members (youtube) or Patreon patrons, simply ask for donations (gofundme, buymeacoffee, etc), or do amazon affiliates and offer gear reviews. In any case, your first step is building an audience.
