What kind of tickets are you talking about? Annual passes with the DVC discount can be purchased thru member services or dvcmember.com.
If you're looking for regular tickets (i.e. 3-10 day, with or without parkhopping, etc.) I recommend you subscribe to the Mousesavers newsletter. They provide links to special deals that are not published on the web.
It depends on what type of tickets you want. DVC members get $100 off Annual Passes and $125 of Premium Annual Passes, you can ourchase these direct from Member Services, through the DVC member website, or in person at WDW. For hopper type or "x number of day" tickets there are any number of websites, including right here in the DIS: http://www.wdwinfo.com/wdwinfo/tickets.htm#hm-tickets-quicklink