Trust me I was searching in there with a magnet for quite a while but from what I can tell it went into a lower hole than I thought and I’m just considering it a part of the truck at this point haha To ace hardware I go today.When I installed mine I dropped one too. I was able to retrieve it by fishing around under the area I dropped it in the frame with a small telescoping-magnet pen. Definitely a pain to get them in there. If I was going to do it again, I would set them in place with a dab of super glue before installing.
If it truly is lost, I don't see any reason a similar washer from the hardware store wouldn't work.