2.5.4 where?

How can I download the 2.5.4 iso of linuxcnc?

I don't think there ever was a 2.5.4 iso, just 2.5. To get to version 2.5.4 you will have to update from 2.5.0.
The old ISOs are stored here
linuxcnc.org/iso/
and this one is 2.5
linuxcnc.org/iso/ubuntu-10.04-linuxcnc3-i386.iso

Alternatively if you already have a version of Linuxcnc installed you can install versions of 2.5 from buildbot.
buildbot.linuxcnc.org/