I bought various sizes at my local auto parts store. They have a big display board of automotive fasteners. I'm away from home for a couple days so can't look up the brand or part number. One problem I had with a few of the originals is that they go into a round hole and would turn with the screw when trying to insert or remove the screw. To solve that issue I bought inserts made for a square hole and used a small square file to reshape the hole in the
fiberglass. Be careful with the screw lengths in some areas as there may her very little space between the inner and outer shell. A previous owner ran a couple curtain rod screws right through the outer shell. Did you try Lowes or Home depot for the inserts. If you don't find anything in a few days send me a PM and I'll give you the name and number of what I used.They come only a few in a little bag.