I'm also searching for a belt clip case that won't be too bulky. However, other than what AT&T has in stores (around $30), I have not found anything yet. I've ordered one from Amazon. The page clearly stated that the case was for the Samsung Infuse 4G, but I returned the case when I got it: it was too small and too short. So, I'm waiting a little while longer. The phone is still very new. The only thing I got was a rubber case by Qubits. It provides some protection and make the phone stick inside your pockets.
Not quite what I was looking for...