The memory controllers have been built into the CPUs for the last couple of years so its about what the CPU will support first. So first find a CPU whose memory controller supports ECC then match up a motherboard.
For AMD you will need an Opterion for Intel a Xeon CPU then find a workstation board. Something like the SR2 for Intel.
Its been a while since I looked at this so someone may know of new speciality non-server boards but on the cpus I am pretty sure I have it right. If not someone will surely let us know. But you can rule out the "normal" enthusiast boards.
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