I think it is like the Henry website says, that they are out of stock at this time. Manufacturing in the US is still running behind but will eventually catch up, which is why what stock they have on hand they are using for new .223's. Hard to sell a new rifle with any magazine. Lucky they had the .308 Long Ranger mags in stock. Just bought a couple recently.
The only way you may be able to find some is used on a place like ebay. When they are back in stock I would buy from Henry. Outlets they do carry them, and they are few, will add anywhere from 25%-100% markup on them, making Henry the best price I have seen at any time.