I've used OpenSubs a few times - https://www.opensubtitles.com/en/home
When they don't have what I'm after I've been ripping my own via DownSub - https://downsub.com/. Not every platform is supported but Viki, IQIYI etc are (but ViP-only episodes and their srt files might be encrypted). If you're on PC for some sites, you can also hit F12, search for "srt", open it in a new window and save it that way. It's been a while since I last did this so I can't quite remember the exact process, but if you do a YouTube search there are some quick and easy tutorials and it's pretty straightforward.
(I know you can do this on MGTV which...isn't the best example as if you think YouTube subs are bad Mango is a step beyond bad for the most part lol. IIRC you can use F12 on Youku too but I can't remember, I haven't been to Youku's site in a while.)
Subshifter - https://subshifter.bitsnbites.eu/ - is also invaluable if you need to resync the srt file due to ads.
I hope this helps, I miss Subscene :(.
