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Current best sites to find videos:
Sita's site: trollvids, now iwara.tv
go to the bottom of the page to change to english
Nicovideo: Search MMD, mikumikudance, R-18, 紳士の社交場 .
Video.fc2: Has video limit views.
Vimeo: There's literally only 3 pages videos when searching mmd r18...

List of free apps/addons (google them) that might help you with downloading vids:
NicoFox
Downloadhelper
Freemake Video Downloader
JDownloader 2 beta

IRC /e/roMMD chatroom
Mostly idling so you might want to leave the tab open and wait for an answer.
#erommd at rizon

For individual models and motions downloads
https://docs.google.com/spreadsheets/d/1_N65CwPcvHNXTOG-E0803aCm31iLknPCwnL2gTHoxBQ/edit?pref=2&pli=1#gid=0