Inter-Tel sells just the HD for EVMC's. The newer ones come with all of the Avdap software on them, you just have to install the correct version.
If your Inter-Tel vendor cannot help you, I would advise calling your closest Inter-Tel Dealer and get a quote for just the HD for an EVMC. Make sure to tell them if it is OS/2 or Linux HD. The HD's are interchangable, but the linux versions have different commands in order to install it.
I would also recommend getting a 40mm fan for the CPU fan. When those start to vibrate, they cause the HD's to go bad shortly after because of the vibration.