Artist Twitter: https://twitter.com/calparas/media
Artist Pawoo: https://pawoo.net/@calparas
Artist Plurk: https://www.plurk.com/calpara
Artist Skeb: https://skeb.jp/@calparas

https://ec.toranoana.jp/tora_r/ec/app/catalog/list?actorKindId=%E4%BD%9C%E5%AE%B6%E2%98%85%E2%98%86%E2%98%85ACTR000000168765
https://www.ruten.com.tw/item/show?21926872200267
https://twitter.com/calparas/status/1138804098772242432
https://www.suruga-ya.jp/product/detail/ZHOA88568?tenpo_cd=
https://doujinrepublic.com/product/product_page_2001049147.html?ref=tag_page&type=tagpage_product

Here's a list of currently unscanned futa on futa doujins that aren't being sold anywhere online as far as I can tell. Most of these doujins were only ever sold physically at conventions which is why I'm listing them on here for the sake of convenience
If you know of any doujin here that's being sold online, feel free to PM me on the forums so I can try to purchase and get it scanned.
If you know of any additional unscanned futa on futa doujin, feel free to PM me so I can put it on the list.
If you know of any additional unscanned futa on futa doujin being sold on a second hand website such as suruga-ya/mandarake, an auction site such as Yahoo Auctions Japan, or any other third-party website outside of melonbooks, toranoana, dlsite, feel free to PM me so I can try to purchase and get it scanned.

LIST OF DOUJINS
P1-4: "宇宙ふたなりアイドル カプリコンツー" by まこっさまん (initially sold at c94)

Artist Pixiv: https://www.pixiv.net/users/27212261

P5-7: "ふたなりちゃんたちのきもちいいこと" from 百乃木富夢 (initially sold at Futaket 13.5)

Artist Pixiv: https://www.pixiv.net/users/27212261
P8-11: Unknown named doujin from 百乃木富夢 (initially sold at Futaket 13)

Artist Twitter: https://twitter.com/fum_hyogetsu

P15-17: "ビュビュッドレッド・オペレーション" by 和泉弥生/Yayoi Kirei (Circle 紀霊なお姉さん) (Sold on May 6 2013 Futaket)

P18: エリちの射精管理はうちの仕事やね by 和泉弥生 (Circle 紀霊なお姉さん) (Sold on May 3 2014 Futaket) (has the same name as a doujin from the author but is a different book)

Artist Twitter: https://twitter.com/kireiyayoi

P19-22: "なかよしフタリ組! 〜ふたなり学級〜" by 和井でぃ and other such authors (anthology book) (initially sold at C87)

Artist Twitter: https://twitter.com/kagikko_yd

P23: "ファンキー DEkamara ルンルンスクール" by したらなな (initially sold at Comiket 92) (unfortunately artist deleted pixiv post before I could get the samples)

Artist Twitter: https://twitter.com/shitaranana

P24-27: "ふたなりJKたちの全裸で体育祭 前編" by LOP (sold at June 9, 2018)

Artist Website: lop4.jimdo.com

P28-31: "ふたなり団長ジータちゃんと銀髪巨乳お姉さんの灼熱のドスケベリゾート" by サークルVertec

Artist Twitter: https://twitter.com/jinbabue/media

P35: "FUJK" (initially sold at futaket 8/released at May 03 2012) (note: not the same doujin as FUJK フタナリ女子高生の放課後)

P36-37: Unknown named doujin (These two images were uploaded on pixiv and stated to be for a cover. Appears to have been between Futaket 10 (May 3, 2014) and Comiket 86 (August 15, 2014))

P38: Unknown Named Doujin sold at Futaket 8 (Artist never put out a cover for said doujin, image might contain title but I don't know how to transcribe Japanese.)

P39: Unknown Named Doujin sold at Futaket 12 (Artist never put out a cover for said doujin, image might contain title but I don't know how to transcribe Japanese.)

P40: Unknown Named Doujin sold at Futaket 12.5 (Artist never put out a cover for said doujin, image might contain title but I don't know how to transcribe Japanese.)

P41: Unknown Named Doujin sold at Futaket 13.5 (Artist never put out a cover for said doujin, image might contain title but I don't know how to transcribe Japanese.)

P42: Unknown Named Doujin sold at Futaket 14 (Artist never put out a cover for said doujin, image might contain title but I don't know how to transcribe Japanese.)

Artist Pixiv: https://www.pixiv.net/en/users/42557/artworks

P43-49: "I don't know how to transcribe Japanese so here's the cover page" by SHIMAD (initially sold at Futaket 10.5)

P50-53: "真 鬼に肉棒" (initially sold at Comiket 89)

P54-58: "鬼に肉棒 2" (initially sold at Comiket 91)

P59-66: "真 鬼に肉棒Revenge" (initially sold at Futaket 13.5)

Artist Pixiv: https://www.pixiv.net/en/users/236443

P67-70: "I don't know how to transcribe Japanese so here's the cover page" by Muruchi (initially sold at Futaket 10.5)

Artist Twitter: https://www.twitter.com/193lovelove

Additional Shit:
https://www.pixiv.net/member_illust.php?mode=medium&illust_id=46416272
https://www.pixiv.net/users/4220971
The Artist Lorica of Circle Frank Factory sold a futa on futa doujin at Futaket 10.5. He has since deleted the pixiv post before I could save any information about it. Yes I am terribly sorry how horrifically vague one time this dude sold a doujin at a random convention and I have no title or cover to show you but trust me bro source it once occurred to me in a dream is but uh let me know if you ever find any additional info to put on here.

Base +11, jd56 +8

今夕是何年

Base +4

我去 肛交部

Base +6