Would be happy to buy, borrow, swap, whatever. Or if people have advice on useful websites, I'd be grateful. Subtitles are easy enough to find but proper dubbing is harder.

Cheers, Sam W

CCTV
There are so many movies with Mandarin dubbing, but only Mandarin subtitles. And you can also look for these movies on 迅雷,土豆,优酷,爱奇艺. But you should type the Chinese names of the movies, and choose the ones with 国语.

www.lele.tv or pptv.com
The newer movies probably won't have chinese dubs. The older ones do.

I saw The terminator dubbed into Chinese and Arnold Schwarzenegger going "我回来" and sounding like a street vendor was too much for me. Never again I swore.