The H8 computers(hell all normal desktops HP has on the market) should use a normal ATX power supplies. They are just various models labeled as HP(Delta/Liteon/ect makes them).
Now something you may have to do on that system is disable secure boot and enable legacy mode if you want to use another video card(Windows 8 systems). Without a proper bios, many cards will not work with that system without these changes.