Kaizoku Fansubs / Forums

Your best bet is probably boot-legged DVDs.

If you live in or close to an Asian-community or anywhere with a high Asian population, then you're off to a good start.
Ebay may have them too. Or online stores that are based in China/Hong Kong.
I'm close to some Chinese malls that have video stores that are stocked with nothing but bootlegged movies, animes etc. for just a couple dollars each.
If you can actually make a trip to China/Hong Kong, then you'll find a gold mine (or if you know someone in the area).

A potential issue though; from my experience some DVDs only tell you that they have "Chinese audio". So you're stuck guessing if you're going to get Cantonese or Mandarin. Some will have both or be more specific with indicating which dialect you're getting, but since you're dealing with low-quality bootlegs, the label might not give you what's promised and you'll get shafted.

Also, the quality can really suck. I generally discourage people from buying from these stores, but if Mandarin audio is what you're looking for, I'm not sure if it'll be available online since they are so cheap to buy anyway.