I looked at all the fasteners amazon had i also looked at a fastener website that specializes in things like this. A christmas tree type push in plug needs to be 11 mm diameter and a length of about 35mm. I have ordered and tried several that I thought may have worked that did not. I have one more package coming in that I hope will be it.Here's a set on Amazon for $9.99 and it has various sizes of the "push rivet" plastic fasteners, 120 total:
Amazon.com: GOOACC GRC-30 120PCS Car Retainer 6.3mm 8mm 9mm 10mm Expansion Screws Replacement Kit Bumper Push Rivet Clips, 1PC Fastener Remover : AutomotiveBuy GOOACC GRC-30 120PCS Car Retainer 6.3mm 8mm 9mm 10mm Expansion Screws Replacement Kit Bumper Push Rivet Clips, 1PC Fastener Remover: Automotive - Amazon.com ✓ FREE DELIVERY possible on eligible purchaseswww.amazon.com
Scroll down into the "related" feed and the choices become too large to comprehend (158 pages of products!).
I just found out you can release the fasteners in by sliding a 5/16 Deepwell socket over them from the rearThe pins you can maybe get at a junkyard but the female in the fender is nearly impossible to get out without them breaking.
I have the same problem and am trying to find a workable solution. I did find if you remove part of the inner fender you can get to the inside of the fender. I may even try carriage bolts and nuts 1/4-20 x 1 1/2 in stainless..