Yesterday I installed two trims on one my sedans that I'm reassembling after painting. To remove the trim, I drilled out the plastic rivets. To reinstall I found some plastic drywall anchors at Home Depot. These came in white plastic, 6 per pkg, and have a 1/4" diameter anchor with a center pin that you tap in to expand the anchor. Once secure you can pull out the center pin if you wish. I did have to enlarge the holes on the window frame, not the trim piece, to 1/4" prior to installing. I did paint the hole prior to installing the anchors.
I'm not sure if the dealer has the original rivets as I did not check. A shop that sells body panels may be a source as they often have various panel connectors. I first thought of using POP rivets but I was concerned about rust and the force required to install.

Erich Zappe
Ottawa Ontario