I lost a couple of plugs and couldn’t find anything locally. I found some cheap replacements on eBay…12 plugs for $8.65 (you can order different quantities). I’ll have extras to give my friends.

12 1 1 4" Round Plastic Tubing Plug End Cap 1 25 inch 11 to 18 Gauge 11 4" | eBay

These are hard plastic vs the original factory soft plastic and fit much tighter in the bumper tube ends.