I purchased brown pop rivets from Home Depot for a gutter project on my home. I suspect they also have white pop rivet options. May have been in the gutter area as opposed to fastener area. It has been 4-5 years ago.
I suspect they are standard diameter, the length is probably fairly short. I have a pretty good assortment with my rivet gun, I would probably just poke around, find a size I like, then find them in white.
Hope this link works, and consider Lowes or Ace Hardware as well.
https://www.homedepot.com/s/pop%2520...20white?NCNI-5
These are for a suspended ceiling, might be just the part.
https://www.homedepot.com/p/SUSPEND-...59-6/305043877