Where can I obtain some of these, preferably pigtails (or even just the ends): Female DC Power Supply Plug to PC Case Fan Power Adapter Cable
Amazon, if not, find specialty stores/companies (Granger type). That's what I'd do.
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@YouInMyEye Good tip, thanks. I quite like Grainger. They're good and also apparently serious about sustainability. https://www.graingeresg.com/