Some contests, especially those with big prizes require special tickets to enter. You can only win these tickets in other contests. Most of all these are contests of one series - tickets for super-contest on final games can be found as prizes in contests on ½ final and ¼ final for these games.
Number of tickets given as prizes in such a contests are limited by the number of winners (top). You can check for prizes list in corresponding tab in contest info page.
List of received tickets can be found in “Profile” page in the “Tickets” block. After you use a ticket to enter a contest it would be marked grey there.
Number of drafts you can send to contest that require tickets is equal the number of tickets you have for that contest. For example, if you have two tickets for super-contest, you can choose two drafts for that contest.
You can not buy or sell any kind of tickets. All received tickets are connected to account of a participant that win’em. If you do not use your ticket to enter a contest it is issued for it can be repealed.