Begin the search for discount tickets on the Disneyland website. It's not likely that you'll find any deals beyond 30% off or so, unless you purchase a package with hotel or airline tickets. But check their website, as they sometimes offer special deals, like discount passes to Southern California residents, group discounts for parties of 10 or more, summer rates for Southern California residents and buy two get two free promotions. Anytime you buy in advance from the website, you save some money and of course time.
Look for reputable places to buy Disneyland tickets in your own neighborhood. Safeway offers advance purchase price Disneyland tickets, which is a slight savings. Costco offers deals through its travel site, but off and on sells "Play 4 Days" tickets. These tickets sell for $140 and allow four days admission to Disneyland or California Adventure. Pay attention to restrictions, though, like blackout dates, expiration dates, or other conditions. These are usually only available to California residents, but you can buy them online. When you get the Disneyand, you have to show them a Southern California ID.
Search reputable websites that sell tickets. Many sources claim that the best place to buy tickets is ARES travel, but it seems that they only offer passes for more than one day and park hopper tickets. Unless you plan on making Disneyland a four- or five-day affair, this won't work for you. Some more popular sites that might have deals are Travelocity, Cheap Tickets and Expedia.
Look into special offers. Disneyland offers discounts to California teachers, students, youth groups, AAA members, residents of Southern California, corporations, military, some government employees and members of the Disney Vacation Club.
Pick up the phone. Disneyland travel agents are very helpful, of course cheerful and actually want you to find the best deal. Ask lots of questions and use your manners! Try calling (714) 520-5060 to see if they can give you some inside information.