Although a DVD/Blu-ray release date has not yet been announced, you can already pre-order season 1 of Dominion on amazon.com. The set comes as either DVDs or Blu-rays. Amazon.com also offers the first season on their Instant Video portal.
Other sources where to watch the show are Syfy Now (not all episodes currently available), Hulu and iTunes.
FYI: Most of the streaming portals are not accessible to non-US viewers due to licensing reasons.