The only way in B/W 1 is through Dream World. It an be accessed through this website by making an account and entering some data about your Pokemon game.
All Pokemon in the Dream World have their Hidden Ability, so don't worry about that.
2 other options are to trade for one with a friend or on the GTS, or trade one over from your own Pokemon B/W 2 that was gotten from a Hidden Grotto (this will take 2 DS's).
Note that some Hidden Abilities are unreleased and cannot be gotten through DW or Hidden Grottos.
EDIT: Pokemon with Hidden Abilities can be received through Pokemon Dream Radar as well.