It would be helpful to know what you drive.
With that said, while you might be able to chase down part numbers online, I think you'll find that OEM fasteners are outrageously expensive, and going by aftermarket part numbers can be challenging, since there's not much standardization among manufacturers. The way most plastic auto fasteners are marketed and sold is by type and dimensions. You can pick a source (say, for example, that big auction website, or that other big website named after a river in South America) and type in a general search like "7mm push clip" or "8mm push clip". You'll usually find clear illustrations giving at least the hole size, or ideally, the dimensions of each of the fastener components, as shown in the attached pic.
You can usually approximate what you need by measuring the existing fasteners and comparing the fastener appearance to the ones offered for sale.
For what it's worth, for the front bumper cover on my '05 Chrysler 300C, I used part number 6957316603040 from Gooac Auto Parts. For the fender liners, I just compared the originals to my selection of fasteners and used what I had in hand, so I can't give you a part number.
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