On this page of the Sons of the Forest guide, you will find information on where to get each of the three keycards available in the game.
The Maintenance keycard is the first of the three keycards you will find during the game. It unlocks access to a series of bunkers scattered around the map. It is also required to obtain the other two keycards.
To get the Maintenance keycard, you need a shovel (you can read about where to find it on this guide page). Once you have it, head to the creek area between the western beach and the mountain in the center of the map.
There you will find two golf carts and geodetic cameras. Keep looking around until you see a shovel icon. Start digging there to find the entrance to the bunker. The keycard is on the table next to the 3D printer.
The VIP keycard is located in the bunker between the west and north beaches. This place is marked with a pulsating dot on the GPS. In the cave, you will find a passage leading to the food complex. To enter it, you must have a Maintenance keycard. You must also have a diving kit to explore the entire area.
The VIP keycard you are looking for is at the very end of the bunker. In the flooded corridor, you will find a door leading to the left. Swim through it to get to the room with cameras - the item you are looking for is here.
The Guest keycard is in the bunker in the western part of the map. It is marked on the GPS with a pulsating dot near the winding stream. You need a Maintenance keycard to enter it. The item you are looking for is on the table in the bar upstairs.