I have usually found them under the name "Well Nuts," and the ones for 1/4" screws will fit in the holes without modification. You should be able to find them for about $.70 each at the local hardware store, but it may take a couple trips to get enough (I think I needed a dozen when I peeled my roof rack off a couple summers ago.)
You'll also need to get new screws when you replace the well nuts, since I seem to recall the OEM well nuts answering to screws with a weird thread. Use brass or CRES button head screws for replacements.
5-90