So each volume of BSD has its own two-pages omake (extra), vol 19 is the latest one, which means that there've been 19 omakes so far. Thus I made this list for those who want to read them ^^

Omake 1, 2, 3, 5, 6, 8, 11, 12, 14, 15: https://yeouya.tumblr.com/tagged/bsd-omake

Omake 4: https://akutagawaprize.tumblr.com/post/135911685094/volume-4-omake-translators-vanillue-raws

Omake 7: https://fukasenanairo.tumblr.com/post/144008075487/ive-never-had-tumblr-eat-my-posts-before-i-guess

Omake 9: https://dazaiscans.tumblr.com/post/144441842130/scans-tantei-bozu-tltypesetting-andy-some

Omake 13: https://akutagawaprize.tumblr.com/post/163934217004/volume-13-omake

Omake 16: https://judaluffy.tumblr.com/post/182085350928/bsd-vol-16-omake-translation

Omake 17: https://chazukekani.tumblr.com/post/187673976948/source-bungo-stray-dogs-vol-17

Omake 19: https://popopretty.tumblr.com/post/622547220873576448/bsd-vol-19-omake-the-omake-this-time-is-really

So the missing ones are omake 10 and omake 18. Omake 10 are the pics of this post, translated and edited by me since I cannot find the English translated one, and guess what? I don't speak Japanese, but luckily BSD is currently published in my country, Việt Nam. I bought every volume and they all had their omake attached. So, yeah, omake 10 here is brought to you in a complicated way through Japanese -> Vietnamese -> English

Unfortunately, I cannot find the English translated version of omake 18, and this vol has not been published in my country yet ;___; I found its raw and will post them in the comment section, hopefully someone speaking Japanese could translate them for us. Please comment too if you found its English translation.