I have a very low opinion of American dubbing in general, although I will not say all of it is bad. Slayers, FMA and even more so Digimon (just think of all the added stuff in 02)... yes, it sucks. Sorry.
If you want Slayers... it used to be in
Boxtorrents, but it's gone now ¬_¬ or I couldn't find it. You can still find the Discography though.
But.... check this
LINK it's 28GB though.
It's mkv, so you can use certain programs like the mkv toolkit to edit out the damned english audio. (which is what I do.)