Essentra Components has lots of useful hardware – small quantities are available at Digikey and the usual sources.
If you need to mount a circuit board at right angles, PCB-MB-01 might be very useful. It may also be useful. These could also be useful for laser-cut acrylic enclosures.
Another item that I came across today are self-tapping screw inserts. These plastic pieces fit in a square hole, and act as both threads for a self-tapping screw and a spacer for a panel or circuit board. I may use these for mounting an easily-removeable bottom panel on an enclosure I'm working on.
Similar parts are available elsewhere, but Essentra is a good source with detailed drawings and ready availability. If you're stuck for a solution, take a look at their vast array if parts.