Obtaining 10.6 Base SDK for Xcode 4.5.2
No, you can't download the 10.6 SDK separately.
Is there a reason you can't use the 10.8 or 10.7 SDK?
Keep in mind that just because you build against the 10.8 SDK, that doesn't mean you can't also have that built application work on 10.8, 10.7 and 10.6. (This is often a cause of confusion among new developers). You control backwards-compatibility through the Deployment Target setting like shown in the image below.
By default, the deployment target is generally set to the same version of OS X as the SDK is, but changing it to 10.6, for example, should allow it to run on a machine with OS X 10.6. (Of course, you should really test to make sure that's the case).
Download xcode with MacOSX10.6.sdk. Now copy MacOSX10.6.sdk inside /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs
Its also working fine with Xcode 5.0.
