I know there is one because I have one. It fits my iPhone 4 in a Griffin Reveal bumper case However, I'm not going to be able to do much to help you find one. I'm using a case that I used with my 3GS which was in a generic hard case, both of which I picked up in a mall in Taunton, MA from one of those little kiosk places. It is a belt clip holster made of leather with a magnetic closure. The only marking on it says "Ultra".
I'd recommend you head for your local mall and hit the kiosk phone case vendors and try a few out.
i have used it with the 3gs and the 4 with switcheasy cases (hard shell case) and with isipio dermashot silicone case
ps the switcheasy cases are pretty nice I had the CapsuleNeo. wraps the phone in a thin silicone to protect from scratches, then install the hard shell over it. i don't see any 4 cases yet though http://www.switcheasy.com/