You could substitute a BC548, BC549, or even a 2N3904 or 2N2222 (might have a different pinout though), for the BC547, and they should all work fine, since they are all NPN types.
A BC107, BC108, or BC109, should also work fine as a substitute for a BC547.
I just checked-out the link you posted and I think that's a great little bit of test gear, looks easy enough to build.