What are these fasteners called, exactly, and where can I buy them? (Robo uses them to mount Raspberry Pi boards in their printers.)
Looks like a panel pin.? Not sure what the exact name is
Bottom entry nylon board supports.
Go look up PCB Standoff
That’s a broad category which covers all types including these.
Use digikey
Surely those could be designed and printed 
gotta try that
Brook
Should be able to buy them off the shelf at most car shops as well.
Oh, weird my most recent comment didn’t post last night – the correct term was actually reverse mounting board supports-- where they enter in the same direction on both boards, versus the conventional supports where the mating surfaces are facing each other. You can find them here: https://www.digikey.com/products/en/hardware-fasteners-accessories/board-supports/578?k=nylon+board+supports&k&pkeyword=nylon+board+supports&pv5=11816&pv5=11815&pv5=12219&pv5=11817&mnonly=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25 – I’ve been staring at supports, standoffs, and spacers for a project lately, so you asked at the right time!
@Joseph_Chiu Thanks, man! That was exceedingly useful info!
Wow, I guess I haven’t looked at a Digikey catalog in years…
You’re welcome! Like I said, I was staring at these things lately, so I knew exactly what it was! 
They are probably in Solidworks library. Should be able to make a model.