Here are some sources for trim clips:
Florida Auto Fastener has eBay store
Depending on the location on the body and degree of corrosion, the existing fasteners maybe can be reused. There is some added risk of marring paint or glass if things do not snap right into place. It would not be an overstatement to call those one-time-use parts.
We have Buick Master Body Parts books for that year to get the original GM part numbers for additional searches and crossovers to other models and makes.
For ornament, front fender, side (aka porthole or ventiport) the Buick part number is 1360152 right, -153 left. EBay shows these available in reproduction from CARS Inc.
What has been, can be again. (Bob Wills, 1942)