I usually buy from Ebook Japan. Easy to use, you can subscribe to ongoing series, no region restriction, and they accept foreign credit cards. honto is easy to use as well, but I don't like their reader. It's a bulky piece of crap. Then there are bookwalker and rakuten books, but I think both require a Japanese IP if you want to buy Sunday manga.
These bonus illustrations come printed in postcards size when you buy the first print edition from Toranoana, animate, or Gamers. As they don't ship anywhere besides Japan you will need either a proxy buyer or a forwarding service. These bonuses sell out pretty fast. The one from Gamers might already be gone, at least they removed any notice of the bonus illustration.
The other possibility would be taking the sample images and photoshopping that text away, if you're good at photoshop or know someone who is.
There are finally some news regarding Matsuena's new work. The original announcement said it would start in winter, but according to some people who get Shonen Sunday early it has been delayed to spring.
Edited by Nacchi, January 12 2018 - 02:37 AM.