Try checking out the "Servers" forum or the "Looking for" forum. Find something interesting and leave your psn name there. People are always trading their psn names or tags on here.
You can send a friend request from your ps3 by searching for a specific psn. Just leave a short message with a little info and demand that they play MC with you OR ELSE! . . . Whoa! Sorry! Just say you like MC and you'll have plenty of friend requests in no time.