First, you will probably be fighting in Khazam at that level ... most people in the 30 range will be there, so it's a safe bet. What you need to do is stand in Kazham and send tells to every party leader in Yhoator asking very politely if they have any openings in their party or IF THEY WILL SOON BECAUSE SOMEONE IS LEAVING. 9/10 times they will immediately ask their party if anyone is leaving and it puts your name in their head.
If all else fails, you can try starting your own party. I had to do that as a Red Mage constantly in the 20-30 range where most noobs still don't know what red mages are capable of. If you can find a WHM ANYWHERE, you can make a group in Kazham.
Thiefs are hard pressed to find groups at that level because many people don't realize the insane damage they can do. The above leader contact method works anywhere worldwide ... you may also think about sitting in Jeuno, doing a /sea all 29-31 and sending a polite message to every party leader that you see.
A good way of doing this is first send a message saying "Excuse me ... " and waiting for a reply (in case they are in battle). Then when they reply, ask: "Do you have any openings in your party for a THF/*? If not, are any of your party members going to be leaving in the next half hour?" Believe me, I've done this many times and I've got into numerous groups doing it. Also, /sea all #-# whm invite is your friend ;)