The key caps are custom to Garmin, I would assume. The switches will just be common-or-garden backlit microswitches (albeit very expensive ones rated for aviation) and those are easy enough to buy (Mouser, DigiKey, RS etc).
Simionic did at one point sell a set of rubber key caps for their bezels to replace the older ones which were plastic. Those are correctly sized for the real Garmin unit, I believe. However they aren’t on their store for sale. You could maybe email them and ask if they would sell you a set?
There’s this project on Thingiverse which has printable button caps but they are for the GNS 430 rather than G1000 and I think they’re slightly different: Thing files for Garmin GNS 430 - Flight Simulator Hardware Interface by allanglen - Thingiverse
See this previous topic: Buttons, Rotary Encoders and Data for a G1000 - General Discussion & Community Support / Home Cockpit Builders - Microsoft Flight Simulator Forums