I couldn't find the holster at the apple or att store. However I was able to find one in leather that fits the I3 & I4 since we have both....MobiMan® Premium- Genuine Leather Vertical Punch Case with Belt Clip for Apple iPhone 2G 3G 3Gs, iPhone 4 4G on Amazon $9.95 + s&h. and yes the phones fit in the case with the shell on the phone.
My dad's got one that works really well. It's the horizontal holster, leather with magnetic flap. I don't recall the brand of it this evening (I'll try to get that tomorrow), but it's a lot like this one on Amazon.com (coincidentally, this one is described as being "for use with shell case").
PS: My dad got the iPhone 4 just a short while back and had the iPhone 3GS before that. He bought a very similar holster for the iPhone 4, but the fit on the two holsters is very different. If one holster is claimed as fitting BOTH, I'd be a bit wary of the claim. It's likely either going to be too tight or too lose on the other phone, but will likely fit one of them just right. I do know that the horizontal leather holster he had for the 3GS was a perfect fit. It did not work for the iPhone 4, so he got another holster for the 4. He uses the new holster like the old one (with iPhone in a shell and phone and shell in the holster). I think he got the 3GS iPhone 4 holster at Fry's (in person you can test the fit). If ordering via Amazon or the like, make sure to read the user reviews since reviewers will likely mention the fit with protective shell/case. The thickness or thinness of the case/shell you're using on the phone could make a big difference in holster fit, too.