can you get the exact width and length from mfg.? If so, local Oreilly's, etc. should have them but be prepared to pay a pretty penny for a set of 2 or 3 of them, like 1 buck each for only 1 plastic push pin. I went into local OReillys auto parts store and counter lady said if does not fit, bring it back and get you rmoney back so worth a shot, but ask first.
Or, do as said and buy an assortment of plastic push pins off of Amazon.com and take your chances buying a pack of 50-250 assorted sizes and hope one fits.
Or, I have not tried it yet on my cars rocker panel being plastic with plastic clips (one having an odd size upfront was missing but used something else laying around up in there for now that holds but not great) went to local Farm and Fleet and think this (although white not black) might hold fairly well being pop toggles. Anyone EVER use these pop toggles before? I think they would work? Hey, for 2 bucks for a pack of 6, worth a shot vs. Oreillys, etc. spending 3 bucks for 2 to 3 black push rivet anchors.
https://www.farmandfleet.com/products/1111395-hillman-pb-pop-toggle-5-8.html