Remember that DC to DC power supplies are much smaller because existing AC PSU has to deal with the transformer aspect.
I worked for an ISP that was also a CLEC back in late 90s / early 2000s. All of the telco side was DC (massive 48v battery banks) - so when we were doing server implementation it made sense to get all DC PSU on the servers. Not sure why DC PSU aren't a more universal option as you'd think UPS providers could easily offer DC output models.
There are options out there, I ran across this [0] looking for typical ATX DC to DC PSU.
