Just to be clear I am not asking how to get parts, or what parts to stock up on. I'm also not asking what parts I should use.
I think everyone who does designs on a regular basis will build up a set of standard parts you pick from.
In my case, for example, I have 2 SMA board-edge connectors I will use (one that actually provides very high performance, the other that is just a cheap but reliable SMA connector for lower frequency control signals). Whenever I need to use a SMA connector in a design, I will grab one of these. Will they be the best possible choice on the market for my application? Probably not, the high performance ones are really quite high performance, and as a result not exactly cheap. However, the time it would take for me to go find a new one, check its performance, design the footprints, etc is probably going to cost a whole lot more than that connector, and thus for prototypes I always use it if I need to be able to trust the performance.
Similar things apply to basic DC/DC converter modules, or certain connectors, or PCB standoffs, protection diodes, LEDs, etc..
What I was wondering is - How did you build up your library? In my case, for most of the connectors, I got recommended them from colleagues. Others I just found them at some point and decided to keep using them. Still others it will be the support provided by the manufacturer (samples, application notes, eval boards, etc)