Where do I get parts?

Where do I get parts? I have a Nest 2nd Generation thermostat that no longer works in heat mode. It thinks it is turning on the furnace, but the signal doesn't reach the furnace. It works fine in cool mode. There are guides to replacing the parts on this site, but no way of diagnosing the correct part to replace or of buying a part or finding where to get parts. ANSWERS:
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