where do I get Rocker panel caps/plugs

[old Chevy guy]

Who knows where to buy replacements for the plastic caps/plugs that go in the holes in the front of the rocker panels? I need some to put in the holes of the panels I put on my cab that Im rebuilding. Also was thinking of using a properly sized hole saw to drill a few (maybe two) holes into the back of the rocker in a few spot so that I would be able to put in some of these caps and then pull them out periotically to clean out and or dump oil inside the rockers as rust prevention. Good idea? opinions? Thanks for any info and or feedback.