Search auction sites for 'assortment kits' and you will find plastic boxes full of a variety of hardware and other equipment.
List of useful assortment kits from local or chinese manufacturers
* stand offs (m3 seems like a good start. m2 is likely a bit small) * crimp connectors * automotive connectors * wire nuts (a kit is the only way to go with these. you never know what size you will need) * nuts and bolts (cheap but good for quick stuff) * washers (i like the huge washers, they are called fender washers) (these are not cheap), also lock washers (not spring washers which even nasa does not like according to https://en.wikipedia.org/wiki/Washer_(hardware))
Just search for "assortment kit". don't ever buy the kits that are in the shape of a circle. those won't stack well in the cabinet.
Keep in mind, some items may be of poor quality. The idea is that these are cheap and ready for quick work. Not to be used in production items.
Harbor Freight stocks a few of these assortment kits. They may be scattered across the store.