Hi,
All available versions are listed here:
https://pve.proxmox.com/wiki/Downloads
As shown on this page
https://pve.proxmox.com/wiki/Downloa...28ISO_Image.29
You can download either via http or bittorrent (although the link to bittorrent appears to be broken at time of writing).
PS just 2 minutes ago I never heard of Proxmox, but Google was nice enough to provide me these information just typing "Proxmox VE 2.3" in the search field.
PPS But From the link to http download I don't see a way to get 2.3, so I suggest you ask them directly