Is the source code for xnu-6153.60.66~39 available? (iOS 13.3 Kernel)
Yes, Apple does still open source the XNU kernel. However, there's usually quite a time lag from the time macOS is released for iOS products and until the corresponding XNU kernel source is publicized.

Therefore you cannot find the exact version you're seeking anywhere. iOS 13.3 is very new, and no public source code has been made available for that kernel version yet.

Usually Apple makes new releases of the XNU kernel source 2-6 times per year. The latest was in June 2019, which is was version 4903.241.1.