Here you will find the official links to buy the games of the mgq saga both original trilogy and paradox plus some other monster girl related games and the ones featured in the collab.

Part 1 EN DLsite:
http://www.dlsite.com/ecchi-eng/work/=/product_id/RE074254.html
Part 2 EN DLsite:
http://www.dlsite.com/ecchi-eng/work/=/product_id/RE087736.html
Part 3 EN DLsite:
http://www.dlsite.com/ecchi-eng/work/=/product_id/RE115697.html
Part 1 JP DLsite:
http://www.dlsite.com/maniax/work/=/product_id/RJ074254.html
Part 2 JP DLsite:
http://www.dlsite.com/maniax/work/=/product_id/RJ087736.html
Part 3 JP DLsite:
http://www.dlsite.com/maniax/work/=/product_id/RJ115697.html

Part 1 EN DLsite: http://www.dlsite.com/ecchi-eng/work/=/product_id/RE150726.html
Part 2 EN DLsite: http://www.dlsite.com/ecchi-eng/work/=/product_id/RE201109.html
Part 1 JP DLsite: http://www.dlsite.com/maniax/work/=/product_id/RJ150726.html
Part 2 JP DLsite: http://www.dlsite.com/maniax/work/=/product_id/RJ201109.html

https://www.dlsite.com/maniax/announce/=/product_id/RJ279436.html
featured in the Collab:
SHIRFT
https://www.dlsite.com/maniax/work/=/product_id/RJ209839.html

The Three Charms
https://www.dlsite.com/ecchi-eng/work/=/product_id/RE241533.html

Succubus Prison ~House of Lewd Demons~
https://www.dlsite.com/maniax/work/=/product_id/RJ174492.html

ROBFS4U
https://www.dlsite.com/maniax/work/=/product_id/RJ137267.html

Succubus Rhapsodia
https://www.dlsite.com/maniax/work/=/product_id/RJ181931.html

Parade Buster
https://www.dlsite.com/maniax/work/=/product_id/RJ176518.html

Succubus Quest
https://www.dlsite.com/maniax/work/=/product_id/RJ026617.html

IMPORTANT NOTE:
Please don’t post in the comments links to download pirate copies of any of the games of the list, if I see one of those, I will delete those comments without warning.