I bought a Kobo Forma recently (in the U.S., from us.kobobooks.com) and logged into it with my Japanese Rakuten account, which has an American credit card on it, and was able to buy books on the Forma.
In my experience with using Japanese stores from overseas, some credit cards work and some don't, seemingly randomly. Rakuten Japan does have actual overseas-address support in their account system at least.
If you're just looking for something to try out, you might consider getting free novels from a web novel site like
http://syosetu.com/
The site itself offers plain text and PDF file downloads, though I'd recommend using kepub conversion downloads through
https://narou.nyanpass.jp/ (just change the format drop-down from "Epub 形式（Kindle以外）" to "KEpub形式 (Kobo用）")