Netgate also provided a VMDK for 2.5.1: https://files.netgate.com/mirror/downloads/pfSense-CE-2.5.1-RELEASE-amd64.vmdk.gz
Netgate typically only hosts the on their official site. Older versions like 2.5.1 are usually moved to archives or third-party repositories.
https://files.netgate.com/mirror/downloads/pfSense-CE-2.5.1-RELEASE-amd64.iso.gz
