For Classic Era the 13 PvP East realms are made up of 2 separate clusters of 5 and 8 realms:
World PvP is rare and typically observed at either the gurubashi arena or arathi highlands for the Tidal Charm trinket. Neither of these clusters receive free transfers. Both of them queue with the rest of the North American region for Classic Era battlegrounds.
I myself play Horde with my guildies on the 5 realm cluster. Give us a /wave if you decide to visit!