The right answer to this is basically: search Google for as long as you care to, and you'll find a good bunch.
Because, really, they're a bit all over the place.
BowlRoll is one, indeed. But a number of them there are passworded, usually the password is available in the description in the main page, sometimes it's not. Also it's in Japanese, you'll probably want a translator to browse that site to make sure you don't miss key words when they only appear in the page's descriptions typed by the author, which is usually typed in Japanese (like 'download' 'password' etc). Otherwise the 'download' is clearly indicated at the bottom of the page in English.
You can also try on DeviantArt, there's a bunch of them available for free, but some of them are just preview pictures and the description links send you on YouTube or on Patreon because they're paid-for animations.
Also, simply search on YouTube itself, you'll find a bunch with links of Motions / Camera / Songs downloads in the video's descriptions; but usually those links send people back to BowlRoll, sometimes on other sites (usually Japanese ones at that).
Really... it's a bit all over the place. You need to spend time on Google and search for days, if not weeks. I've been doing that myself On and Off for about a year and even though I did find a high enough number (all available for free, so far) only a few of them were the "full" package. Which means that yes, sometimes I do find the actual Motion (animation) files, but the music track associated with the Motion is missing, or the Camera animation file is missing, and then starts the long search to find those missing individual files.
Because, ideally, you want the "full" pack to use with the MMDPlayer (Motion file + Camera file + Music file, so that when you select the folder using the plugin it all comes in together fine and dandy in one pack and everything is auto-selected properly by the plugin). Sure, you CAN select files individually if you don't have packs, per se; but still... it's better to just find everything in one go when you search for it on the internet.
Good luck, it's a long burn process.
