Those that I have as paper books were bought in Japan or in Thailand during my lucky trips to either of these countries. Any of my friends travelling there are also requested to bring whatever book marked as bestseller in BookOff as a souvenir 
Alternatively my friends often use CDJapan to order real books since amazon-jp bans sending anything to Russia (no clue why).
And well, most books I have are just digital versions bought in one of the e-stores. I use BookLive, for example.
Also this thread here may help: https://community.wanikani.com/t/resources-for-starting-to-read-japanese-content/31644