Where can I download the ovm_layering_agent extension kit?

You can find it further down the list on this forum - Easier Tests with OVM layering.

However, you may want to check out the UVM page on layering which shows an alternative approach which is easier to use and doesn’t require additional components:

http://verificationacademy.com/uvm-ovm/Sequences/Layering

The approach outlined should work on OVM.