Someone else asked the same thing less than two months ago and got a good reply with a helpful download link. Please go further and further back in time in these two links:

Requests: https://ecchi.iwara.tv/forums/requests
Help/questions: https://ecchi.iwara.tv/forums/helpquestions

After a while, you will find a request or question that is similar to yours. Then please just click the download link in that reply.

[Update: Here is it, forum post 46 days older than yours: https://ecchi.iwara.tv/forums/need-imarine-project-official-stuff-download ]

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(The original site iMarine automatically deletes all motions after exactly one year. The one you want is from 2017 so it was deleted in 2018.)

Here are all the motion dance of iMarine-Project:
https://www.deviantart.com/shyguymmd/art/MMD-iMarine-Project-Models-Moti...