Opening day ticket always go on sale in advance on a Saturday in February I think and usually open up around 9 or 10am (i don't remember the exact date but they will announce it a few weeks in advance). Usually once the ticket lines go open (both phone lines and online), they sell out in half an hour or less, and oftentimes the server is hard to get through. That is probably you're best options if you don't want to buy at a premium but otherwise you will probably have to rely on Stub-hub or Ebay and will likely end up spending close to $50 a ticket or more just for view level seats from my experience.
Good luck. The experience is definitely worth it because Cincinnati opening day is something to remember.
Edit: It looks like things might have changed as of last year and it may now be a ticket lottery based on this article. http://www.wcpo.com/dpp/sports/baseb...online-lottery
Sorry if I provided wrong information but that is how it was a few years ago ( I have been a season ticket holder for the past 3 years and haven't had to worry about purchasing them separately)