Although site is mostly in Japanese, browser 'auto-translate' works well.
A lot of models are freely available to down-load.
Some need password often found on linked nicovideo page.
Many need sign-in.
Previously, you registered by jumping through complex, multi-lingual hoops, give all sorts of ID. I never managed to join...
Now, you may sign in with twitter or Facebook...
https://bowlroll.net/file/tag/MikuMikuDance
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PMX Editor, free on DA, can export items as un-rigged OBJ+MTL. Handy for props, sets etc.
https://www.deviantart.com/inochi-p...-v1-0-766313588
Care: OBJ is mirrored. If this matters, use editor's 'mirror' tool prior to export...
Truebones' $$ 'PMX to FBX' utility can convert rigged PMD/PMX to Poser-compatible rigged FBX.
Note this first version only ports 'hues', not texture calls...