Email me for our modified versions of Gadget-2, modified to include self-interacting dark matter physics or late-time decays of dark matter. Note that our group now works with a modified version of the AREPO code (by Mark Vogelsberger) because it includes more physics and scales better with the number of compute nodes.
At some point (I am writing this August 2023), I will curate links for good places to start learning how to run simulations.