TEDx events are locally and independently organized, so the best way to get tickets varies from event to event. Generally, tickets can be purchased from the event page on TED.com.
Find an event page by searching the TEDx event listing page.
If you can't find tickets on the TED.com event page, try searching for the event on the web. Many TEDx events have their own websites, Facebook pages, and social media accounts, which contain more information.