I emailed MrMetric asking them to stock those thin (DIN562; 1.8mm thick) square nuts used in loads of 3D printer parts and they came through! If you’re in the US this is the only retailer I’ve found that sells them (in quantities >10 and/or at a reasonable price). More info in the comments.

http://ift.tt/2hJRRzF http://ift.tt/2AaZiGN