there
https://seiga.nicovideo.jp/seiga/im2973807 (School gymnasium)

https://seiga.nicovideo.jp/seiga/im6876655 (Dangan ronpa's Locker room and gym)

https://seiga.nicovideo.jp/seiga/im7615628 (Fitness set A)

https://seiga.nicovideo.jp/seiga/im8044550 (Fitness set B)

https://seiga.nicovideo.jp/seiga/im5367699
(You can build a gym stage(a.k.a Fitness club) by placing fitness sets A and B)

You're welcome sir.
I'm glad I could help you. keep up the good work :)

Ideas that come to mind immediately

For example:
1.Try to reproduce "Intercrural sex(or Buttjob) → cowgirl position or reverse cowgirl position on a lying bench press table by diverting squat motion" (If it is an existing motion, the model coordinates may penetrate, so it may be necessary to correct it with camera work. I think it goes very well with the motion created and distributed by Ademar440.)

2.Try incorporating cutscenes that interfere with using "Gentlemen's hands(紳士ハンド)" or some "Toys" during weight-workout in mid-waist position .

If you want more ideas, you may be able to get more ideas by watching the video on Youtube about "Workouts to Get a Bubble butt".

However, since there are already pioneers in story works using a series of gym stages, it may be necessary to be careful not to overlap situations and motions.

That's them
https://ecchi.iwara.tv/users/mantis-x
https://ecchi.iwara.tv/users/89189
https://ecchi.iwara.tv/users/%E5%8F%A4%E4%BB%A3%E5%B8%83
https://ecchi.iwara.tv/users/%E5%B2%B3%E9%85%94

Hmmm...so the list of accessory models you are looking for would include this one?

https://bowlroll.net/file/1444　←(Detail: Barbell, dumbbell, wide bench, flat bench, power rack. BUT VERY OLD DATA)

https://seiga.nicovideo.jp/seiga/im6372222 (Pull-up stand)

https://seiga.nicovideo.jp/seiga/im1919306 (Treadmill)

https://seiga.nicovideo.jp/seiga/im5047195

https://seiga.nicovideo.jp/seiga/im5227202

https://seiga.nicovideo.jp/seiga/im9804638

I was able to find some data for "indoor aerobics" that is (probably) free to download and use.
But it probably does not have a ".x" extension, so you would need to load the data in Metasequoia or Blender, then change the extension so that you can use it in MMD and save the file.

Also, according to the article written by the author, the 3D model data for the pedal part where the feet are placed is not included, so you need to make the parts for that part in 3D.
http://hamstern.air-nifty.com/comic_studio3d/2010/06/3d-8cdc.html

Thanks

i gg and has many types, try search it and take a look of those