Which dub are you looking for? There are a few:
Original Japanse dub
Steamline English dub
Disney 1998 dub
Disney 2010 dub
The streamline one can be found with the tag [REVO] online, the Disney 1998 dub is found on the first DVD release in the US (2003) and the VHS release (1998). The Disney 2010 dub is found on anything newer than that. The original Japanese dub can be found on any DVD or Blu-Ray release.
I am personally looking for a copy of the 1998 dub, but with the Blu-Ray quality.