Hmm, looks like that assortment is not available from Harbor Freight (at least not from the online store). I bought it in a brick-and-mortar store probably a year ago.
I don't think Harbor Freight actually makes these kits.
Looks like these can be found on ebay:
https://www.ebay.com/itm/125pc-Rubber-Grommet-Firewall-Wire-Gasket-Solid-Hole-Plug-Assortment-Set/192919251868
This seller wants $10. I do not think I spent that much when I bought it in the store.
A little searching on ebay or google would probably find a better deal.
Maybe start with
https://www.google.com/search?q=0.5+inch+closed+grommet&tbm=shop